Yeah, so to access most of the higher Arch features, you have to unlock the immutable filesystem Valve's put in place, then initialize and populate pacman-keys. Then you can pacman -Sy to get the repos updated. But you've heard right, running Windows software is only 2 steps more with Proton. You still can add the direct EXE to Steam (I have VGA Golf on my SD card), then you make sure you've enabled Steam Play for unsupported titles, and then you can choose a Proton version to run the game in.
With regards to running it in Proton, after enabling Hidden Files, the path of /home/deck/.local/share/Steam/steamapps/compatdata/########/pfx/drive_c/users/steamuser/AppData/Roaming/Trederia/ got me to the config files, no logs in sight.
I tried again with running the Linux build, in terminal, it states "./vga_golf: error while loading shared libraries: libBulletDynamics.so.3.24: cannot open shared object file: No such file or directory" which is in /libs/
Yeah, reading the Flatpak guide makes my head spin a little. Alongside the software (or a .sh file representing the script), you need a runtime which will act as the environment, and then your deps are "bundled" as needed, depending on what your software would demand. The guide for AppImage is a lot easier, it seems you can use CMake as normal, change a few options e.g., rename to AppDir, explicitly define CMAKE_INSTALL_PREFIX=/usr, and then use linuxdeploy to wrap it up into an AppImage. It may just work with the release you have. I'll power on my desktop and try it.
EDIT: My desktop runs Linux Mint and the highest version of libbullet right now in Ubuntu repos is 3.06, so I would have to compile 3.24 to even think of running VGA Golf. This is before even packaging it into an AppImage.