I think that Lutris should have "Proton - Experimental" option in Wine versions, instead of needing setting up a Custom executable. You might need to install some package.
But if I'm mistaken, I guess the Custom executable should lead to a binary, not a directory. Search in home/bob/.steam/steam/steamapps/common/Proton - Experimental/bin
for something like wine
or proton
And btw, I think you are mixing two things: wine prefix is directory with a fake directory structure that resembles Windows. Preferably it should be created when installing the application, so setup creates directories in Program Files
, registry, etc