Based on all you’ve said, Carleton seems to be your best fit culturally. I say this as a recent Pomona alumnus.
Carleton is distinguished for undergraduate teaching and has been #1 by US News on that metric for the last few years. If you look at professor averages at RateMyProfessors, Carleton teachers are rated a bit higher (4.16/5- I haven’t seen better for any school) than those at Amherst (4.06), Pomona (3.96), Brown (3.88), and Bowdoin (3.8).
Carleton, as warblersrule pointed out, is a top producer of both chemistry and biology PhDs. Both among LACs and nationally, period. Just since 2000, Carleton has yielded 127 graduate NSF fellows and 14 Goldwater Scholars. These are measures supporting that Carleton can produce distinguished students in the field and that academically it is as good as it gets.
I agree that Carleton has a special nature. It’s definitely not the least-laid back. It’s not as preppy as Amherst, Bowdoin, or Brown. It is academically rigorous, yes, but it’s not a pressure cooker. The students are caring/collaborative and the community feels especially tight-knit, even relative to other LACs. As much as I love Pomona, I have to acknowledge that I have been more wowed by the sort of conversations and interconnections my Carleton friends seem to have (on social media) than just about any other college out there. If you felt that Carleton gave a less competitive impression than Pomona, that is pretty telling.
“I want to be surrounded by a generally happy student body. Learning is absolutely my favorite thing to do. I also want a college with a strong sense of community.” This reveals it all. This is literally Carleton’s unique culture among the top liberal art colleges. Go visit Carleton and if you fall in love/it is affordable for you, go for early decision.