Yeah, I think you're right and that corruption games are just sex tropes older than feudalism coupled with "modern" video game mechanics, mostly of the "growing up numbers by repeating actions" variety.
I actually managed to find an interview from Akabur where he states Princess Trainer is not inspired by any H-game in particular, just basic fetishes and mainstream RPGs.
I'm starting to see a pattern appearing in my researches:
Game is released and gets popular (Sengoku Rance, Corruption of Champions, Princess Trainer)
Game inspires very directly other games, bordering on pure plagiarism sometimes
Game might or might not influence other, more distinctive, games, but not in a way that's immediately obvious within the context of such a superficial analysis
Which is to be expected I guess. If history would simply unfold itself in a couple Google searches, the life of historians would be far simpler.
Also, I'm wondering if I'm not still missing some influential Flash H-games from the early 2000s, on Newgrounds or elsewhere, now lost to the sands of time but that used to be a big deal, just for completionism's sake.