I found a Brazilian community server getting ready to open and it is focused on AllTheMons with a big arena duel system.
They made their own serverside mod called CobblemonArena so you can join without installing anything and just fight.
You can do direct challenges or jump into matchmaking with commands. When a battle starts the arena closes up in a circle so nobody can mess with it.
Duels run with a 10 minute timer and 6 Pokemon with free switching. You win by having more Pokemon still alive and ties go to whoever has more total HP.
There is no automatic healing after a duel so your team leaves the same way it entered and every match feels serious. People can also watch as spectators and the results post to Discord with a ranking bot.
They are still finishing development and the server is not open yet but they say they are close.
