It has been a while since I played Divinity: Original Sin 2, and I'm still in Act 1 of BG3, but from memory:

D:OS2 has fewer bugs and better performance. This isn't surprising, of course, since it has had more time for polish.

From what I've seen so far, BG3 has:

• More balanced battle mechanics. In particular, battles aren't dominated by excessive surface/cloud effects or telekinetic barrel drops, and I haven't yet had a fight where I felt unfairly disadvantaged by my party lacking one specific ability.
• Far fewer instances of the targeting UI lying to me and causing frustration in battle.
• More world to explore.
• Richer lore, as told through books and journals all over the world. It reminds me a bit of Elder Scrolls in this respect.
• More interesting writing. (This might be subjective, but I would be surprised if most people disagreed.)
• More character depth.
• More immersive voice acting. (For example, the voice actors almost always understand the context of their lines. They often didn't in D:OS 2, which I found distracting.)
• Better character animation (outside of cut scenes, some of which are a bit awkward).

The gameplay is indeed similar, of course, as it's the same kind of game, from the same studio, using a revision of the same engine. But this one is IMHO better in almost every respect, and I think I'm more likely to play it again when I'm done.

i am somewhat suspicious that people think Baldur’s gate is some novel masterpiece

Novel? Not really, except maybe to people who haven't played its predecessor, or good BioWare games, or D&D. More like an improvement on what came before it.

when really it’s that Divinity is super under rated

Where in the world have you seen D:OS2 underrated? I sure haven't.

and relatively unknown by comparison.

Well, yes, that's to be expected. D:OS2 didn't have half a century of role playing game history or Hasbro's marketing budget behind it.