So, I decided to switch to Arch from PopOS!, because Linux Hardware suggested improved compatibility with my FCS. After installing SDL2, installing a fresh copy of the game, installing CKAN, installing Express Rp-1 & the FBW mod, and launching the game, I saw no change:
I tried tinkering with the UI scaling to see if that would do anything, but no help. The window is pinned to that location and I cannot manipulate it in any way.
After looking through KSP.log, I found that still:
DllNotFoundException: libSDL2-2.14
The SDL version that was installed automatically by pacman was sdl2-2.28.5-1, so I found:
sudo pacman -U https://archive.archlinux.org/repos/2020/12/22/extra/os/x86_64/sdl2-2.0.14-1-x86_64.pkg.tar.zst
which is hopefully the correct version. After trying to test again, I got the same result as the above image, so no change. It is supposedly the correct version, but I guess the name of it is what's causing issues. Looking for libSDL2-2.so, I found libSDL2-2.0.so.0, as a link to libSDL2-2.0.so.0.2800.5. They already existed in ~/.steam/bin32/steam-runtime/usr/lib. I'm guessing I can just copy the files over somewhere? Or where do I go from here?
While looking for where to potentially copy the files, I found additional copies in /lib/libSDL2-2.0.so, as a link to libSDL2-2.0.so.0.14.0. I found a CMAKE file, if that helps, somehow.
I decided to make a copy of the file libSDL2-2.0.so and rename the copy as libSDL 2-2.so to fully conform with the guide. After testing, I still get the same result; a pinned window and an error saying DLLNotFound. My guess is there's either a pathing issue somewhere or it's still the incorrect version.