The thaumaturgy rules are really not engineer-proof. I could cite some rules to explain why putting a landmine on a bullet is not really workable but it'd be beside the point. If a player wants to break thaumaturgy they probably can, and the best answer to ask them politely not to.
To clarify to Londo, you can't put a ward on a moving item. You could set up a magically enhanced bullet on gun targetied at a door that goes off when a door is open, that would be legit.
You can't magically enhance a bullet to put in your gun and carry it around everywhere with you. Well, you can...but you use the enchanted item crafting rules.
I blame Pathfinder; but my group is pretty optimised minded. That's not to say they're not great roleplayers, but they'd like to have a solid mechanical base to run that off.
Thank you for clarifying how power is relevant to certain things like blocks and maneuvers. That always confused me.
My remaining concerns are thus
1) Landmines: as I understand it, you don't need evocation to put evocation abilities into wards. So the Landmine ability could constantly trickle a powerful spell into the ward. My group would definitely start every day making some thaumaturgy wards before heading out (sacrifice a handful chickens etc.). I believe there's a section that says a ward needs concentration so likely you couldn't have multiple of them, but I don't want super powerful landmines to rule the game.
This will almost never come into play. Are they sacrafiicing chickens everyday to protect their home? Fine. Let them do that. In fact, you can assume they have a ward with a wardflame and a warning system and at least one landmine attached to their home at all times.
Will they have one in the abandoned factory that has been taken over by red court vampires? no, definitely not. If they want to reinforce the room where they are sleeping, that would take time and maybe they're being harassed by vampires as they're doing it.
If they have to defend their house, just assume they have those defenses and make it part of the adventure. Maybe they are doing a ritual to prevent some terrible fate for a loved one. They choose to do that in their ward-protected house. The ward becomes a plot device and/or mechanic that gives them time to get their ritual off on time. Assuming they do the ritual properly. It's a great way to create tension.
edit: you don't need to concentrate on wards. They last until sunrise unless you put extra shifts in duration. So you may only have to kill chickens 1/month....
2) Evocation: this is a dense question, but how does one defend with evocation? I know you can create Blocks (shields and stuff) and maintain the spell, but the everyday block mechanic. Is it another Conviction + Discipline roll, and if so does it induce a stress? Or does a spell caster either choose to keep up a block or have to use something else like athletics?
There's no default defense with evocation. You could take a stunt that lets you defend with lore. It would use up a stress to activate it each time...
They could use enchanted items that go off when attacked. It would be based on their crafting rating and would be considered a block against attacks (or an athletics skill replacement).
There is a house-rule that people use that lets you cast a spell as a 'dodge' action. It takes a stress and the spell only works for that one attack (not for the full round).
BTW this community is great
thanks, and welcome.