Sorry, but I beg to differ... And not meaning to start a waifu war. I like all the girls. Just a technical debate about character development

I think Jill has had both shining and awkward moments since the beginning; she didn't change too much. I know the Tybalt's thing took a toll on her, but, in the end, she managed it well. Anyway, even if you say it's true (and, I guess, in this case, there'll be only statistical truths, like X% think so; Y%, don't), I don't think this change was deliberate; I bet Mr. DPC wrote her like he always wrote, thinking he was writing her like always, and, if you feel this way wasn't due to a deliberate decision but due to Mr. DPC shortcomings in his writing skills (not that he's bad... in the AVN universe he's probably top 5). And she's been planned to be a virgin since the beginning. DPC has to tick all archetype boxes.
About Sage, honestly... I don't think it was gradual... She was the best girl in season I (I'd say, from ep. 1-5, because the changes started from ep. 6 on -- and not only for her) and then she became a bore (partially due to some choices Mr. DPC forced upon us, like, "nOt WaNtInG tO bE a ReBoUnD bOy"). Then she emerges like some kind of closeted intellectual, being all smart and all when doing research for her senior thesis, and also being willing to help people. A total U-Turn with no subtlety.
Maya, for me, gets more awkward as time passes (because... see spoiler box below), and I can't wait to get rid of her to have a Josy-only path because... Well... Josy is Josy! Alady!