You may need to limit the number of cores you're giving the game. According to Proton DB, it's got a gold rating on Linux

https://www.protondb.com/app/799600

I'd try adding --cores 4 to your launch options and reducing it from there if you have issues still. Also seems like Proton 10.0-3 is the ideal version.

If you need assistance getting those setup:

1. In your steam library, right click on Cosmoteer in your games list and click on properties

2. Under Compatibility, make sure Force Use of Specific Compatibility Tool is checked. Select Proton 10.0-3 as your Proton version. If you haven't used that version on a game before, it may need to download and that'll take a bit, which is why I'm having you do this first.

3. Under the first section in properties (I'm typing this all from memory, apologies), there should be a section that says launch options. In that text box, type %command% --cores 4

4. Launch the game. If it works, great! If not, try reducing the number after --cores in your launch options to 2. If it continues not working, try switching to Proton experimental. If it continues not working, I'm out of ideas unfortunately.