Nevermind, the problem wasn't with my card, my kernel, or my petitboot. I booted up fedora workstation and gfx worked fine. The problem was just with KDE (and other DEs) not accepting multiple graphics drivers as described elsewhere on the wiki.
Before, I wasn't able to log in and set my system up without enabling the ast, so blacklisting it was problematic. But I was talking to some guy on IRC, he reminded me that you can access petitboot over serial. That's good enough for my setup. I can just use the VGA disable jumper and turn the PC on with autoboot most of the time.
The one problem I have right now is that I use genkernel's initramfs to unlock my LUKS storage, so I need to build that with amdgpu or something, otherwise I have to do it over serial. But yeah having serial interface to petitboot is awesome because it gets handed over to the main OS with kexec.
Too much information, I know. but I like to just follow up on this sort of stuff online just in case its useful to someone else. You wouldn't believe how much time ive spent on a wild goose chase flashing firmware. And all this stuff gets fed into a search engine or an LLM anyways...