Can't run BL2 on Linux via Steam Proton

I am using Linux Mint 19.2 and would really like to play the new BL2 DLC.

Since the DLC hasn’t come to Linux, not yet anyway, I have to install the Windows version via Steam’s Proton. Trouble is I can’t get it to work. I went to properties and ticked “Force the use of a specific Steam Play compatibility tool”, I’ve tried Proton 4.11-8, 4.2-9, but the game doesn’t even launch.

Am I missing something? Any help would be appreciated.
Thank you!