Yeah I've installed Bnet on the Steam Deck using Lutris and I agree it's not a very intuitive process. Lutris does its best but there are still some things you will have to look up.

There's another script that's rising in popularity, NonSteamLaunchers, which aims to automate this process for you. I haven't tried it but the people who have, have said that it's easy. I'd suggest giving it a try.

D4 on gamepad is VERY playable. I'm glad Blizzard learned from their experience of releasing D3 on consoles and implemented those on D4. Obviously MnK is still faster for some actions, particularly inventory management, but you can really play the game without feeling like you're hampering yourself.