I'm likely late to the party on this, but I added the MTGA client to Steam as a non-Steam application today, so that I could lounge on my couch and try it over the Link.
Turns out that some kind stranger has already built a great controller profile for it (Magic Arena Beta v2 by OMG WOW), complete with buttons for passing priority, ending the turn, etc. It feels great, especially on the Steam Controller, which probably bodes well for an eventual console release.
If you've got a Steam Link, give it a shot.
EDIT - I'm not sure how to export the profile as .txt for non-Steam Controller owners, so I took a photo of it. Here: https://imgur.com/a/k3Yg0
EDIT 2 - Added credit to the maker of the great Steam Controller profile, Magic Arena Beta v2 by OMG WOW.
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