Man, I picked up 'Saint Magic Power is Omnipotent' on a total whim after seeing the anime first, and I've gotta say, the books are a different beast. The anime skims over a ton of the little moments that make Sei's character so interesting—her quiet research into potions, her awkwardness with the knights, that subtle frustration of being a modern woman stuck in a rigid fantasy world. The novels are slower, for sure, but that's kind of the point. It's a comfort read about finding your place and building something real, not just another isekai power fantasy. I'd recommend starting with volume one even if you've seen the show; there's a lot of internal monologue the anime just couldn't fit.
That said, if you're looking for constant action or a super intense plot, you might find the pace a bit glacial. It's very much a slice-of-life story with magic. My sister tried it and dropped it after two volumes, calling it 'too fluffy,' which, fair. But for me, that's the appeal. After a long day, it's exactly the kind of gentle, low-stakes story I want to unwind with.