From the little testing I did, XMage supports commander and there were no unsupported cards in the 5 or 6 decks Ive tried. You do have to use the beta release though, the stable release is like 3 years old.
It's still a little clunky, but handling priority is generally a nightmare in a 4 person game. I think that's part of why Arena doesn't even bother trying. Waiting for 4 people to pass priority is annoyingly long when it happens like 12 times per turn. Games would take all day to complete.
I'm aware there are unofficial solutions like XMage which is why I mentioned "official". As for handling priority, they really need something like the "auto" option from Master Duel where there is some rules engine that tries to figure out when you would want to hold priority. For example, I'll generally only want to tap a mana dork on my turn or if I have some instant speed ability/card in hand to use that mana. There are exceptions, but the auto system just needs to be good enough that I can anticipate how it'll work so I can turn it off as needed. Master Duel also has a cool feature where you can press down your mouse to always ask for a response if you know your opponent will do something soon that you care about.
It's still a little clunky, but handling priority is generally a nightmare in a 4 person game. I think that's part of why Arena doesn't even bother trying. Waiting for 4 people to pass priority is annoyingly long when it happens like 12 times per turn. Games would take all day to complete.