Thanks a ton for this guide! I got it all working on Debian 12.

A couple of differences I'll point out (in case it helps anyone else):

• I was using the system's default apt-version of steam, but I needed the flatpak-version of protontricks to follow the guide because otherwise protontricks would crash. I think this is because the apt version of protontricks is too far behind.
  • sudo flatpak install flathub com.github.Matoking.protontricks installed protontricks
  • flatpak install com.github.tchx84.Flatseal installed flatseal, which allowed me to give protontricks additional filesystem permissions (otherwise protontricks wouldn't detect the steam games)
  • Open flatseal GUI, locate protontricks and give it more filesystem permissions
• I also couldn't seem to get Steam to run the Untapped.gg Companion.exe executable after the install step. The problem seemed to be due to the . and white space characters in the name, so I renamed the executable to untapped_gg_companion.exe and then it worked
• Finally, on my Debian laptop I require PROTON_USE_WINED3D=1 %command% in my Steam launch options for MTGA - otherwise the game wont launch. So my final launch option including this and the OP's was: PROTON_USE_WINED3D=1 PROTON_REMOTE_DEBUG_CMD="/media/<username>/home/SteamLibrary/steamapps/compatdata/2141910/pfx/drive_c/users/steamuser/AppData/Local/Programs/untapped-companion/untapped_gg_companion.exe" %command%

Thanks again @pwalker@discuss.tchncs.de!!! Very much appreciated!!