It’s heavy on the D&D 5e rulebook, if you have any level of familiarity with that it will all come naturally.

If you don’t, that’s ok too, they did a really good job describing rolls, saving throws, attack rolls, etc, in the guided tooltips.

If you haven't played a game like this, there is a learning curve. I've been playing stuff like this for decades and there's still lots to explore, in purely mechanical terms, with BG3.

That said, it is very much worth the initial investment. I found playing Divinity 2: original sin very much prepared me for BG3. You could start there a little more cheaply before delving into this expensive title. At least then you can see if you enjoy this style of game.

Being familiar with dnd 5e will give you a head start in the basics. Things like how ability scores and combat/social interactions can play out. But not necessary. They've changed quite a few minor things that deviate from the tabletop game that I've found myself having to actually unlearn dnd.

Monks are actually useable. Went with a 4 elements subclass. So I have a mix of spell-like abilities for range, and can still smack things with my staff and fist.

The main thing is, being aware of the micromanaging you'll have to do in combat. Don't be afraid to use consumables. And explore! I'm still only in act 1.( I've had pre release for over a year, so I'm taking it slow to see what changes are in the actual release).

If you're passingly familiar with 5E D&D, you should be pretty much good to go. The CRPG-ness of it shouldn't be too much to get your head around if you know the underlying system at all.

You can do what I did: turn down the difficulty until you get the hang of it. It's adjustable during the game, so you can crank it back up later.