I tried running it with steamtinkerlaunch wich does open fenix.exe but it seems to crash as it immediately exits when i choose to run only custom command. The log of stl is of no help either

So 8. Okt 18:26:07 CEST 2023 INFO - setRunProtonFromUseProton - Updating RUNPROTON for USEPROTON 'proton-unknown-999' from '/home/xxx/.steam/root/steamapps/common/SteamLinuxRuntime_sniper/_v2-entry-point --verb=waitforexitandrun -- /home/xxx/.steam/root/steamapps/common/Proton - Experimental/proton' to '/home/xxx/.steam/root/steamapps/common/Proton - Experimental/proton'
So 8. Okt 18:26:07 CEST 2023 INFO - setRunProtonFromUseProton - Set RUNPROTON to '/home/xxx/.steam/root/steamapps/common/Proton - Experimental/proton'
So 8. Okt 18:26:07 CEST 2023 INFO - extProtonRun - Continuing with RUNPROTON='/home/xxx/.steam/root/steamapps/common/Proton - Experimental/proton'
So 8. Okt 18:26:07 CEST 2023 INFO - extProtonRun - No external program args here it seems
So 8. Okt 18:26:07 CEST 2023 INFO - extProtonRun - Starting '/home/xxx/.steam/root/steamapps/compatdata/1250410/pfx/drive_c/Program Files/FenixSim A320/Fenix.exe' forked into the background
So 8. Okt 18:26:07 CEST 2023 INFO - restoreOrgVars - Restoring previously cleared Variables
So 8. Okt 18:26:07 CEST 2023 INFO - launchCustomProg - Changing pwd into previous directory
So 8. Okt 18:26:07 CEST 2023 INFO - checkCustomLaunch - ONLY_CUSTOMCMD is set to 1 means only custom program '/home/xxx/.steam/root/steamapps/compatdata/1250410/pfx/drive_c/Program Files/FenixSim A320/Fenix.exe' is supposed to start - exiting here
So 8. Okt 18:26:07 CEST 2023 INFO - checkCustomLaunch - ## CUSTOMCMD STOPPED after '1' seconds playtime
So 8. Okt 18:26:07 CEST 2023 INFO - closeSTL - closing STL
So 8. Okt 18:26:07 CEST 2023 INFO - updateConfigEntry - Emptying 'CUSTOMCMD' for '/home/xxx/.config/steamtinkerlaunch/default_template.conf'
So 8. Okt 18:26:07 CEST 2023 INFO - updateConfigEntry - Option 'CUSTOMCMD' is updated with the new value 'echo' in config 'default_template.conf'
So 8. Okt 18:26:07 CEST 2023 INFO - closeSTL - Creating '/dev/shm/steamtinkerlaunch/steamtinkerlaunch-closing.tmp'
So 8. Okt 18:26:07 CEST 2023 INFO - checkPlayTime - The game was not started via 'steamtinkerlaunch' but directly from Steam - skipping Crash Requester
So 8. Okt 18:26:07 CEST 2023 INFO - closeSTL - Game 'Microsoft Flight Simulator (1250410)' exited - cleaning up custom processes if necessary
So 8. Okt 18:26:07 CEST 2023 INFO - closeTrayIcon - Closing TrayIcon '169695'
So 8. Okt 18:26:08 CEST 2023 INFO - closeSTL - Removing '/dev/shm/steamtinkerlaunch/steamtinkerlaunch-closing.tmp'

Does https://www.protondb.com/app/1250410 help?