Extremely pleased with how my refactor went. I removed tons of explicit states from my state machine and have most stuff working through simple yield() statements waiting for animations to complete. The top of my _process function is a bit ugly with all the guard clauses, but it's so much easier to work with. I can effectively code the whole thing linearly now even though a lot of stuff is actually being handled across multiple frames.

I also added a nice new animation for playing a card. Instead of just disappearing, the card slides to the middle of the screen, pauses for a second while the effect is applied, and then flies off the top of the screen. It slows down gameplay a bit since you can't just mash through your whole turn in less than a second, but it looks really slick.