No, the event in the pool if you stay with Bella in Episode 4 is also significant. The MC stops his constant flirting with her for a moment and she decides to continue of her own volition. It's a vital part of her development.
Unfortunately, that becomes a problem because it's also entirely optional. As a result, the Episode 5 lewd scene in her sauna winds up feeling disjointed. If you got the pool scene, it feels like regression - that even after Bella admits what she wants she still insists she didn't. But if you skipped the pool scene it's even worse, because it looks like the MC is just refusing to take no for an answer and keeps harassing Bella until she gives him what he wants. I suspect this is because DPC didn't want to restrict Bella's arc to only one route in Episode 4, but the end result is a badly sprained narrative.
So IMHO it's not that there aren't any other significant developments in Bella's arc, it's that the game has to pretend those development aren't significant.