Theory: I think Gregory will stop appearing starting with Eloise's season, and won't return until his own.

I don't know what his story is like, since I haven't read the books, but considering that all of his older brothers went to Oxford, I think it would make sense for him to go out of sight for a while, and then come back later when it's his turn to do his story. I think that would also help a lot with the audience taking his season seriously and not looking at it oddly, since Gregory and Hyacinth started out as the two babies of the story who we've seen grow and mature over the seasons. But if we keep seeing them back to back, without having any maturation in their story (i.e. more mature plots for both of them) when it comes time to see them in a semi-erotic scene with them, like the ones this show handles, it might seem weird because we've seen Gregory throughout the entire series and at this point he's like a little brother to the audience as well. But if he goes away to college and doesn't come back until two seasons later (aka: possibly 4/5 years later in real chronological terms), he'll come back as a new character: older, more mature, with life experiences gained and without this childish image that we all clearly still have of him and Hya, because of the way that, for example, in this sub we always comment on how older they both look already and all that.