I wrote something up a few weeks back that covers many of the missable/ secret scenes. The TLDR of it was each book has around maybe 5-10% of the scenes missable. I think each book (besides book 1) also has a meaningful choice that has specific scenes (bk2 love choice of girl, bk2 morality ending, bk3 love with the lost girls, bk4 slave when you rescue Korra, and if you cum inside some girls will be pregnant for the endings, oh and bk2 love ending choice effects bk4 love slightly). Just play it slow and click each thing before moving on and you shouldnt miss much. A trick if you get stuck is to use the skip function (but make sure it only skips seen texts) that way you can click everyone very fast and find who you need to talk to (because it wont skip the unseen text).
TLDR: No theres not that much missable content. It looks like I wrote lots but theres like hundreds of scenes in this game.
Edit Oh this is important, each route is very different. Book 1 has just small differences in the routes, but make sure you play both routes for bks 2-4! Theres dozens of extra scenes/ content in the different routes..
