The earlier suggestion is solid, but I'd actually push back a tiny bit on length expectations for that one if you're being strict about the 400-page limit. That said, *Radiance* by Grace Draven is exactly what you're looking for - it's a standalone paranormal romance fantasy that sits comfortably under 400 pages and has genuine character development alongside the romance. The worldbuilding is lean but effective, and it doesn't feel rushed despite being shorter.
If you're willing to go slightly longer (like 450-480 pages), *The Shadows Between Us* by Tricia Levenseller is another solid pick. It's darker, with actual stakes and a compelling dynamic between the leads. The pacing is snappy enough that it doesn't feel bloated, and it wraps up completely.
For something a bit different, *Heartless* by Marissa Meyer - if you haven't read it already - is a retelling that works as a standalone prequel to Alice in Wonderland, around 350 pages. The romance builds naturally without taking over the whole plot, and Meyer's got a talent for making compressed timelines feel earned. All three of these give you that fantasy atmosphere and emotional payoff without the commitment of a 1000-page first book in a series. The key difference from the massive epics is they trust their stories don't need to be overstuffed.