Games launch from steam and seem to work, but I’m getting between 0 and 10 fps on the title screen of Kerbal Space Program.

Something is definitely off on your system. I've a 7900 XTX (the slightly-higher-end version of that card), and while I don't have the box in front of me, it definitely runs at at least reasonable (60fps+) rates at 2560x1440 on KSP. Might do well above that, dunno. It's definitely not herky-jerky to the level you're seeing, though.

Are you using Wayland or Xorg?

If you run radeontop (in Debian trixie, package radeontop) it should tell you various load characteristics. There isn't a GPU-agnostic utility to do this, unless things have changed since last I've looked -- Nvidia and AMD both have their own utilities.

I kinda regret not going with Nvidia at this point

Unless you're aiming for AI stuff, where there are some significant benefits, like a large userbase and support for transformers, I'd probably recommend AMD for Linux use.

EDIT:

If you run glxinfo on either Xorg (or Wayland, since the emulation layer will handle it), package mesa-utils on Debian trixie, it'll tell you what OpenGL is trying to use. If you're using hardware-accelerated stuff, you'll get something like this:

Vendor: AMD (0x1002)
Device: AMD Radeon Graphics (radeonsi, gfx1103_r1, LLVM 19.1.4, DRM 3.59, 6.12.11-amd64) (0x1900)

OpenGL renderer string: AMD Radeon Graphics (radeonsi, gfx1103_r1, LLVM 19.1.4, DRM 3.59, 6.12.11-amd64)

That's been the quick-thumb-in-the-wind test to know whether hardware 3d acceleration is running since just about forever. KSP probably doesn't actually use OpenGL -- I'd guess that it's probably using DirectX going through some emulation layer in Proton to Vulkan -- but if you've got something wonky like no usable 3D driver support for your GPU, that'll show it up.

EDIT2: There's also vulkaninfo in (package vulkan-tools in Debian trixie). It'll give you something like:

GPU id : 0 (AMD Radeon Graphics (RADV GFX1103_R1)):

EDIT3: If you're using Xorg and that doesn't show hardware acceleration in use, then the next thing that I'd probably look at is /var/log/Xorg.0.log to see what Xorg is saying regarding your GPU. I don't know much about diagnosing Wayland issues, as I've not been using it for all that long. The kernel log may also have interesting messages information (as root, journalctl -kb or dmesg) if the problem is at the kernel level.