Tbh, I can see the game ending with S4 tops.
But, seeing how MC attracts people like shit attracts flies, it is not impossible to believe that Bella/Jill/MC and close friends had a dinner and shit happened. But also, could be a flashback, the birth of her persona, so to speak.
Overall, I think that the setups are nearing the end, and from then, it's gonna go into resolutions and conflicts.
Season 3 seems like a slow setup phase for S4 to finish it. That's just a personal feeling I'm getting.
Hes not gonna end the game as long as hes making gangbusters of money. Way too risky that the follow up title doesnt deliver in terms of catching the audience.
In terms of timeline, the MC is barely started, so he could stretch it forever, with characters leaving college and new ones coming in etc. Usually games also end when characters get together, which is where the game is already at. But he could chose to actually use the possible staleness of relationships in these games to bring in new chars on those routes, expand their individual stories beyond the college (other friends, family, etc) etc.
There is no rule that these games have to end, they usually just do because people lose interest, have no new ideas or drop off. If he sees this as a challenge in terms of writing and he doesnt want to explore a different settings he could stretch it to the 10y development mark or whatnot. Hes unlikely gonna make more money with any other project, school/highschool/magical school settings are notoriously popular, even in other media like books and the whole litRPG/Wuxia genre kinda grew from it (in terms of growth and learning (getting stronger, better) as a central aspect of satisfaction).
The structure from Episode 9 also speaks for not having a set plan. Thats why the initial reaction was relatively negative, because episode 9 is absolutely not what youd expect to follow onto the emotional end of episode 8, it kinda sets off in a newish direction almost. Im fairly sure he brainstormed what he wanted to do during the long development time (would explain why it took so long aswell) and didnt follow a set plan he had in mind 2 years ago for the characters or overarching story. If he just continued with what was expected it would speak more for wanting to end "soon" imho.