Diablo 4 Steam reviews put Blizzard’s RPG in the ‘mixed’ category less than a day after the D4 Steam launch, as player numbers struggle on Valve’s platform too.
As nice as it would be to have it on steam there is just no incentive to rebuy it. It doesn't even take much extra work to get the battle net version running on a steam deck.
I think the guide I had originally followed was actually for World of Warcraft so I don't see why you wouldnt be able to apply that to Diablo 3 - but the issue with D3 on the deck would ultimately probably be that the PC version of the game has no official controller support.