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Given the Exp. Share can't be turned off I think it'd be better to simply give the Gym Leaders larger teams to compensate hence why we start at four. I've also shifted Bea-Allister & Gordie-Melony into Double Battles because I can't be bothered to make separate gym battles for them plus it'd get the player more used to the format in time for Raihan.
Added also a theoretical Elite Four in a sense although it's an Elite Eight I guess in this case which covers the remaining 8 Types (Electric/Poison, Normal/Psychic, Bug/Flying, Ground/Steel).
As for the rival battles, I don't think Bede or Marnie should be as restricted by the gym type they eventually fall under in a battle before they become the gym leader of that gym hence why their teams have a good number of Non-Fairy & Non-Dark Types respectively. Hop's Team I just gave more variety to, I don't even know why they gave him a Pincurchin but regardless, four members of his team change based on your starter, you have the standard Grass-Fire-Water core that many rivals have but also this tertiary Bug-Ground-Electric element to make the battle more varied, Dubwool because it's Hop and Corviknight because fuck it why not.
Leon's battle also shifts according to starter, I dropped the Type Advantage starter and gave that to Hop because Rivals that you're supposed to consider a challenge yet pick the starter weak to yours are the dumbest shit to ever exist. Anyways Leon's team always consists of Hydreigon, G. Darmanitan, Charizard & Dragapult regardless of your starter with Dragapult being the highest levelled but Charizard still being his G-Max that way he effectively has two aces. First Variant is a Galar Pokemon either Mr. Rime, Barraskewda or Flapple and the second variant is a Non-Galar Pokemon either Vikavolt, Gigalith or Raichu.
If I had access to all 890 Pokemon there's be some I'd want to change mainly due to overlap but that's just how it is with Galar introducing an low number of Pokemon.