Without any mods the game is in 4:3 aspect ratio due to its age (2005). To get it to be 16:9 at 1080p you need to install JSGME (or equivalent mod manager) in the game directory and activate a widescreen mod. The one I went with is the Ahnenerbe WideGui 1920 x 1080 Final option which you can find easily on subsim.com by googling or searching in there. I also installed GWX. GWX adds many mods to the game, and whether its a memory issue (this game came out at a time when games were limited to 2GB memory usage, and as technology advanced had to be patched to 4GB - this is handled automatically by proton nowadays on linux) or something else, when run with any fairly modern version of proton it would CTD whenever loading into a career patrol. Training and single missions ran fine. I tested it with versions 8 and 9 of proton. Version 4 did allow you to enter into the patrol, but it didnt seem to play nicely with the widegui mod. Eventually I settled on proton version 6.3 and it seems to be working just fine.

Note: A handy tool to be able to install windows software that is not the game within the games directory is SteamTinkerLaunch. Follow the instructions on its github to set it up as your compatibility layer, and then you can run one-time-commands from its interface (run the JSGME.exe installer, or the GWX installer)