It looks like steam's proton is attempting to point the game at the system's dotnet installation if present. This understandably causes it to crash immediately upon launch. Thankfully, Steam's first launch script installs the correct dotnet version to the wine prefix, and the 'DOTNET_PREFIX' environment variable lets you instead point proton at this version - Fixing the issue. Here, it was 3.1.8. Expect this path to change with future updates.