That sounds fun tbh I've done some yugioh rp stuff and have done stuff like this. We did BO1 for time reasons. But for a elite4 league - just for expediency- I would say g1 is normal, but if a league is of known decks, then you could pre-side each other game against the known decklist.

I also think it'd be smthn like BO1 (best of one, right?) it's just that for pokemon you can change your team up between every trainer but you're locked to your team at the E4 (1/2)