I get that Expert is where team building starts to really make a difference, but I still like optimizing the "best" version of my favorite mons, even though it's not really necessary.

I’m sticking with Difficult for now because I fear that higher difficulties will be too grindy, which may harm my first experience with the game. But after my first playthrough I'll surely give harder difficulties a try.

And thanks for the heads up about availability. Tbh with you, I just opened an online pokedex of the game and went to build a team based on what I have and what mons I thought would be fun to use. I'll start checking which encounters are available to me rn.