@plibbity There are a LOT of things that have changed since Gen 3, most importantly the Physical/Special split, how EXP is calculated, and the Mega Evolutions/Z-Moves.

Ultra Sun/Moon is a great place to start, though. Especially if you skip original Sun/Moon; there's some things that were taken out but don't feel "missing" in the Ultra versions of the games, if you don't know they were there.