During the times when it hangs, this is the window presented by the game:

Amazing what happened; the game developer looked at the complaint and fixed the bug! Now the game works on Linux. That gives me a lot of hope! https://steamcommunity.com/app/810500/discussions/0/2985287984615866234/
This issue can be closed, I think. I bought the game again and confirm that it works by adding the "-noradio" option!
proton 4.11-13x1 2020-12proton 5.13-4x1 2020-12proton experimentalx1 2020-12PROTON_NO_ESYNC=1x1 2020-12PROTON_NO_FSYNC=1x1 2020-12
Compatibility Report
System Information
I confirm:
Symptoms
Game starts, presents a black screen and then either hangs forever or crashes back to SteamVR
Reproduction
1 - Call SteamVR
2 - Once inside SteamVR, call the game
3 - it starts by fading to black then either crashes or hangs forever.
Sending the log for Proton 5.13-4 (no extra variables) and 4.11-13 (no fsync and no esync). If you need the others please say
steam-810500-proton-5.13-4.log
steam-810500-proton-4.11-13-no-fsync-and-esync.zip