Divinity: Original Sin 2: Larian doesn't know what 'original' means

[Keltest]

I couldn't help but notice the "Skills" are kind of limited in number.

Again, that changes as you go up in level. If you talk to some NPCs around camp, you can eventually identify spellbook vendors for different skill trees. Stingtail, IIRC sells pyrokinetic spellbooks for example, while that other absentminded lizard who's name I cant remember sells hydrosophist.