I agree that improvement destruction could definitely use an overhaul, but I'm wary of some of these suggestions.
Hypothetical
I'm a C20, so I have a maximum capacity of 300 ships. Say I have 300 ships and I have gotten my opponent to 0. I have declared an attrition war with tactician war policy and gained superiority in all three fields. Under your proposal, I could have a modifier of around 300% if they have 1,000 infra and around 337.5% is they have 2,000 infra. That is borderline overpowered modifiers. And that's on top of the already 500-700 infra damage I would be doing (dependent on infra levels).
My Feedback/Opinion
I like, and agree with, the modifier based on war type. It just makes sense in my opinion.
If the modifier based on infra damage were to be implemented, I would suggest raising the number from 50 infra. Possibly 100 to 150 to make it a bit more balanced.
Perfectly fine with the consequences of zero infra listed.
I'm 50/50 on the missile/nuke one. Would need more information. What are the proposed odds for each roll? Any other modifiers? Do nukes have a higher chance than missiles? Etcetera.
I don't agree with the trifecta bonus. It's unnecessary and a bit too much, in my opinion. Kicking someone while they're down is only fun to an extent.
I think if you want to lower the odds of Tactician and Guardian, keep them at the same level (like they are now). Though I could see an argument for having Guardian lower so they don't completely cancel each other out, but the other proposed modifiers would make up for it anyway. There's a debate to be had here.