IT WORKS NOW! I will need time to run additional tests, but the gist of my solution was:
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Backport llvm-18 from sid following the guide you linked at https://wiki.debian.org/SimpleBackportCreation
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After compiling and installing all those deb files, I then installed the "jammy" version of amdgpu-install_6.0.60002-1.deb from https://www.amd.com/en/support/linux-drivers
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Downloaded the latest kernel sources from https://git.kernel.org/pub/scm/linux/kernel/git/firmware/linux-firmware.git, and simply copied all the files from its lib/firmware/amdgpu folder into my system's /lib/firmware/amdgpu. Got that idea from https://discussion.fedoraproject.org/t/amdgpu-doesnt-seem-to-function-with-navi-31-rx-7900-xtx/72647
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sudo update-initramfs -u && sudo reboot
I'm not totally sure it step 3 was sane or necessary. Perhaps the missing piece before that was that I needed to manually update my initramfs? I've tried like a million things at this point and my system is dirty, so I will probably roll back to my snapshot from before all of this and attempt to re-do it with the minimal steps, when I have time.
Anyway, I was able to run a real-world OpenCL benchmark, and it's crazy-fast compared to my old GTX 1080. Actually a bigger difference than I expected. Like 6x.
THANKS FOR THE HELP!
I recently upgraded to a 7900 XTX on Debian stable, as well. I'm running the newest kernel from Debian's backports repo (6.6, I think), and I didn't have that same problem.
I did have other problems with OpenCL, though. I made a thread about this and solved it with some trouble. Check my post history if you're interested. I hope it helps. I can take a closer look at my now-working system for comparison if you have further issues.