First of all, follow the link[github.com] and read carefully the section where to unpack:

"f it does not show up in the list, ensure you extracted the folder in the correct location and that the directory looks something like this "/home/deck/.steam/steam/compatibilitytools.d/DAI-Proton"."

I didn't have the compatibility tools.d directory on my steamdeck, so I had to create it.

After unpacking files from the archive to this directory, reboot to be sure, and then in the settings where you previously selected the proton version, select DAI-Proton

N.B: Before starting the game and checking, change string GstInput string.GamePad 0 to GstInput.GamePad 1 in the ProfileOptions_profile file (/home/deck/.steam/steam/steamapps/compatdata/<your game index>/pfx/drive_c/users/steamuser/Documents/BioWare/Dragon Age Inquisition/Save/)

That's all, now you can set official controller layout in steamdeck control setting and start DA: I