Let's say I managed to max out my starter (and other Pokemon I became champion with) out to level 100, by beating the game, the Sword/Shield royalty arc, the Isle of Armor, and Crown Tundra, but want to start fresh with a new starter (Scorbunny or Sobble) without creating a second account/doing the story all over again.
The only thing I can think of is battling random wild Pokemon, since there are no more regular trainers to gain exp. from until my team is strong enough to battle each Cafe Master, then strong enough to battle Breeder Chloe, Morimoto, Marnie, IoA Rival, Mustard, Honey, and Peony, all without spamming the Champion Cup (Galar's "Elite Four", the final thing you do to beat the game).
I want the experience of training/using as many Gen 8 Pokemon as possible (as I was able to do in Gen 6).