Still playing my Ulta Moon, trying to complete things i've left behind, when i was playing it 2 years ago, like clearing all trainers, exploring wormholes, IV breeding, etc.
Unfortunatelly, the post game content to do in loops are:
Pokemon League: Infinite
Battle Buffet: Once per day
Battle GameFREAK guys , Morimoto and Iwao. Once per day
KANTONIAN Gym. Once per day.
Battle Tree, infinite.
The good part in Ultra Sun/Moon, which i discovered accidentaly recently, in the Single and Double formats, not the SUPER though, there is no level cap, neither banned Pokemon, so it's a good way to use your legendraies, and to avoid the CPU cheating from Super Challenges.