I'm concerned that if we start dealing with stress and consequences as written, the player builds will start to become focused on spike damage, weapon values and anything else that bumps those attack rolls at the expense of declarations and create an advantage actions.

What's good for the goose and all that.

I've got a player in my game that can dish out weapon:7 with a skill of 6, and that's not a spellcaster (he can do spells, too, but those are usually even weaker). I've had him pitched against a similar character, and they fought for 10-12 exchanges, without anyone really taking damage (even though we had a few good hits on both sides). We eventually decided to end the fight on a draw.

Toughness and good defensive skills can really do a lot. If you don't hit, the weapon rating never even comes into play.

Other than that, stacking aspects to get a good hit in is most often the way to go. It's different to other games, where you usually zerg down an opponent. Here, if the attack is big enough, you can take out the opponent with one good hit. Usually, you don't have that kind of firepower with one shot, so you set up the circumstances to make it so. That's what maneuvers and free invokes are for.

I can tell! Its very complicated along those lines as its almost trying to get my player to pick and choose what he wants.

That's working as intended.

If he wants a creature or if he wants, essentially, environmental effects. How would that work short term versus long term, in the sense of evothaum. Is there a particular element or power he should take to help him?

Well, the sponsored magic version would be more complicated but also more versatile. You can do anything with it, as long as you can describe how your magic can do it.
The modular abilities version is more work up front and less versatile, but it is a lot easier to use, since you can avoid the magic rules altogether.

With the evothaum version, you'd basically just say "I play this trap card, which binds an opponent when he attacks me", which would basically just be an evocation block without any fuss. But he could also just solve a lot of problems with magic by having the right cards available. It's not so much about any element, the deck of cards would have its own sort of magic, outside of the standard elements.

For the sponsored magic version, you'd just need to take sponsored magic (deck of magical cards) and think up an agenda for the deck. Maybe a former magician resides in the deck, maybe it was created by someone that wants it to be used in a certain way, etc. That agenda can be used to deal with debt. You could also think about using different skills than the regular ones. Performance would for example fit a deck of cards from a magician.

For the modular abilities version, we'd need to set up the modular abilities first. I'm not sure at the moment how much refresh you'll play with, if it was mentioned earlier, I couldn't find it. Let's stay low and assume a feet in the water campaign.
Since the deck of cards is an item, we can go with an item of power discount, albeit a small one. The basic write up would look like this:

Deck of Cards (Item of Power) [-4]
 Modular Abilities [-2]
  3 Form points [-3]
 small/concealable [+1]

So having the deck like this costs 4 refresh. The next step would be to list the cards. I'm not too familiar with Yugioh, so I'll just go with something generic.

Small red Dragon [-3]
 Breath Weapon (fire) [-2]
 How to tame your dragon (Stunt): Use presence to use breath weapon, when you can order your Small red Dragon to set something on fire.

So any time you draw the "Small red Dragon" card, the 3 Form points from "Modular Abilities" get filled with the powers and stunts on the "Small red Dragon" card, listed above. What happens in the game is that the character draws and plays the card, a small, red dragon appears, and he can order it around to do stuff, which will mostly be setting things on fire. This under the assumption that you will do a lot of summoning and your presence skill would be better than your weapons skill, which seems appropriate for the kind of character you describe.

Now you just need to create a list of cards he has in his deck. All of them can have powers worth 3 or less refresh, and he can only ever use one of the cards at a time. Before he can use another card, he would have to dismiss the old one and draw the new one, which costs him an action (it's what "changing your powers" takes under modular abilities). You can also add new cards throughout the campaign, whenever you feel like it might be appropriate. You don't even have to look for a reason, it's a card he always had in his possession, he just didn't use it before. That way, you don't have to have everything written out up front, a good deck can have quite a lot of cards, I imagine.

That sounds like it could also be a sort of sponsored magic thing. Mainly, because it allows "thaumaturgy at evocation's speed and methods" or evothaum for short. Something like being able to make drawings come to life for a short time.

Then, there is a difference between the means and the end, which is important in Fate. Usually, summoning the creature will not be the end, it is the means by which you do something. You summon a frightening creature to scare someone. You summon a strong creature to break through a door. You summon a flying creature to quickly move from A to B. And so forth. All of that can be done with the "simple task" thaumaturgy rules. Evothaum allows you to do this as evocation as well, not just as a longer ritual.
But since you'd have access to both evocation and ritual, you'd be able todo attack spells etc. as well, the full range.

Then again, you could model this a number of other ways, as well. For example with "modular abilities". This costs 2 points of refresh and as many more as you like. The more you pay, the more points you have to choose from. So if you pay 4 refresh total for modular abilities, you could have powers worth 2 points of refresh to switch around. You could then make a list of cards you can draw and have a list of powers you get with that card. For example, drawing a small dragon could grant you the "breath weapon" power, because you can order the dragon you summoned to throw around fire.
If you've got the desired cards in physical form, just make a deck and a list which card gives which powers and when he wants to change the power, he doesn't choose which card he gets, he has to draw it from the deck.
I would probably grant him the "human form/involuntary change" rebate for that as well.

I hope you can at least somewhat follow me. This is where it gets complicated.

Well, technically speaking, no matter the backstory Im giving them, they would be a mermaid. But, they do have a human form. So, technically speaking, if I casted as a mermaid, I could get away with it correct?

The wardens are usually quite trigger happy. If you do something against the laws or against a human being, they will probably not think too long. On the other hand, they'll need to find you first.

If you are part of a court, you'll have protection by that court, and the wardens will probably not just hunt you down. But they might ask your superiors to keep an eye on you, or your superiors might do so themselves, if you behave too out of line.

Though, maybe your superiors want you to behave exactly like that, and they put a lot of red tape between you and the wardens to let you keep doing what you're doing.

To that end, you could take sponsored magic instead, with the limitation that it only allows you thaumaturgy stuff. It would be pretty much like thaumaturgy regarding the rules for potions, but you'd have a sponsor added to that. A sponsor gives you an agenda, something the sponsor will push you to do, even if you might not want it or realize it. And it allows you to take sponsor debt in order to do something. For example, increasing the power of a potion on the fly.

Well, Thaumaturgy gives you 2 focus item slots which translate to 4 enchanted item and thus potion slots. That's 4 potions per session, as it were, at a strength of your lore skill. If you put your free specialization, which comes with thaumaturgy, on potions, you get potions at a strength of Lore+1.

You can buy refinement, which will give you 4 potion slots for 1 refresh. You can spend a potion slot on a new potion or to get 2 additional uses on an existing one. I've explained this in more detail in the kitchen witch thread I linked to earlier.

Support sounds like having a cooler full of potions that increase the other players could be your stick. Depending on your GM, some potions could give a character a minor power for a short time. Like a potion of flight could give a character the "wings" power.

To do so, you'd take thaumaturgy and a ton of refinement for potion slots. Lore should be your highest skill.

Any potion would have a power based on your lore. In a submerged game, that would be a base of power:5. You could take a potion specialization for your free specialization and make that 6.

You get 2 free focus item slots, which translates to 4 potion slots. Each refresh put into refinement gives you 4 more potion slots. You can spend a potion slot to get a new potion or add 2 uses to an existing potion.

Most of your potions should probably be maneuvers, since those can give benefits to others. For example, you can have an eagle-eye-potion that would give a sniper a benefit when shooting at a distance (he gets a free invoke on the "eagle eye" aspect after drinking the potion, which he can use for +2 or for a reroll).

You could go all out for a total of 28 potion slots. You can leave some of them free and use the "potion declaration" rule. In that case, you can ask to have a potion with you that you don't have announced previously. The GM will then give you a difficulty based on how plausible this might be, and you roll Lore to beat that. If you do, you have that potion with you, the potion follows the usual rules (as if you had it with you the whole time). Or you can spend a Fate point to have the potion without a roll (if you go for the roll and fail, that option is void, though you can use the FP on the roll).

To give a character a minor power, you can do the same, a maneuver potion, but instead of using the free invoke for a +2, you use it to pay off the temporary power (Your Story page 92, yellow box). Some GMs might not be willing to allow free invokes instead of Fate points, so you'd have to ask your GM if they are ok with it. We've had quite a bit of discussion about this on the board here as well.

Ok, I think that's enough wall of text. If you need some more information or clarification, just call me. I'm in the book.

No, wait, that was someone else.

Alternatively, if you really do not want to limit its use, so that it can always be used, then try attaching it to a free compel rather than a mechanical disadvantage.

The "Listening" Stunt for investigation does something very similar to that:
Listening: You can tune out your other senses and simply focus on your hearing, making you
able to pick up faint sounds with astonishing clarity. Gain a +4 on your Investigation roll
when Listening; however, your Alertness drops to Terrible while doing so, due to the exclusion
of your other senses. If a conflict begins while you are Listening, it will take a full exchange
for your Alertness to return to normal once you stop.

And that's what happens when you paste and just hit send...

Anyhow. It's a cool idea, but I feel like it should be a stunt in most cases. Unless it's a stunt that each of your players takes, in that case you could just add it as a rule for all. The stunt basically gives you an opportunity to codify when a specific character might use it. I kind of like that, since you are forced to take it for a specific thing, and the gun nut won't be able to do this with swords and vice versa.

That would be a Stunt.  The maneuver I propose doesn't raise the attack, and more importantly, it doesn't negate a target's Full Defense.

Still, the idea is quite similar and could easily be adjusted to do extra damage instead of granting a bonus.