Assuming you completed the Pokedex in ORAS and transferred all relevant Pokémon to USUM, that would mean you would only have Gen 7 Pokémon to worry about. After that, it should be pretty easy to just obtain all the new Pokémon, barring certain mythicals, by just playing both versions regularly, trading version exclusives as necessary. The only hiccup is Cosmog/Cosmoem. Okay, technically not, since you can obtain it in both versions, but it's annoyingly well hidden. I couldn't find it without a guide, which I'm also including here for that reason.
If you want to go full completionist, then as far as mythicals are concerned, Magearna's event is perpetual, so you can obtain that one no problem, but Zeraora is unreleased (although it'll probably be available in the near future, considering the movie coming out soon). Marshadow is a trickier case, since its event has ended, and you won't be able to get one unless you personally know someone willing to give you one, since its event ribbon only allows it to be traded through a link trade, banning all legit Marshadows from the GTS and Wonder Trade.
