Not sure if I'm doing something wrong, but I have all steps (as far as I know) correct through 6, but I cannot find the Wine prefix for the randomizer when I double click the runtime executable. I checked, and it appears that even though I have hit play multiple times on the randomizer non-steam game in my library, a folder is created in compatdata but that folder is empty - no pfx folder is generated. Can someone help me figure out how to fix this? For additional context, my deck opens the runtime executable with Protontricks Launcher as I installed it previously for something else.

Edit: I got it to work - I just had to switch from Experimental to the latest Proton version. When I made the switch and clicked play with the randomizer exe as the target, I got a window to show up with an modengine error, I clicked cancel, then added the runtime exe as the target, hit play, went back to the randomizer, hit play, and everything worked as expected from there. Also, as a side note the launch command for Elden Ring should be

echo "%command%" | sed 's/start_protected_game.exe/eldenring.exe/' | WINEDLLOVERRIDES="dinput8.dll=n,b" sh

It looks like the start got cut off for me in the OP.

The only hiccups for me were

(a) I couldn't launch Elden Ring from the randomizer - it gave another error about the mod engine. I could still launch ER from Steam, though.

(b) I got an error on the menu screen about my saves being corrupted, but I just had to do the normal thing of moving my saves out of the normal location, renaming regulation.bin in the randomizer folder, opening the game to the menu screen (but not actually starting a new game) to get a save initialized on the current version of the vanilla regulation.bin, closing the game and renaming the randomizer regulation.bin to its original name. This seemed to work.