To be fair, it's not the type of game that requires high framerates to play.

What settings are good for BG3? I just got it and am wondering how best to tweak it

That's disappointing. Haven't tested it on the deck yet as I'm waiting for new screen to arrive. It runs great on my moderate gaming PC with a 6600xt. Runs at 1080p ultra and hovers between 100 and 120fps. Usually closer to 120.

I fiddled with it a fair bit, and ultimately decided to just stick with the "Medium" default. With that, it's pretty stable, with a few dips, but given the turn-based combat etc, it doesn't really bother me.

(But that said, I'm an old-school PC gamer. I played DooM on an 8 MHz 386 at sub-10 FPS. I had to "lead" my shots to play. So my personal standards for game performance may be a little low.)

I found that turning off VSync and setting the FPS limit to 30, along with capping the Steam Deck frame rate at 30 from the UI, gives me good results. I also turned off FSR 1.0, turned on fidelityFX sharpening, and slid the slider all the way to the right. My settings are mostly on low, except for texture quality, which I set to medium.

My FPS isn't always a steady 30, but the game stays very playable, even when it drops to the low 20s. I'm not super picky about performance.

I kinda struggle to get about an hour and a half of gameplay on a full charge (my battery health is at 97%).

That's normal, I have been using fsr from steam deck's menu and got it running at 30 fps for the most part, it can still go below 30 but it is very playable.