The answer used to be John the Ripper, but I’m a decade out of date on this stuff, so it might not be any more.

It’s a 125gb game, what kind of disk were you expecting it to ship on?

On protondb one guy mentioned that a full uninstall reinstall (not “check game files”) helped him with a “wouldn’t get past loading screen” issue, but that doesn’t quite sound the same as your issue.

Have you tried proton experimental?