Vocastar explained an important difference between losing and finishing a route: a bad end leaves the player character as an NPC, while reaching the end reveals the player as human. That makes route survival part of the story outcome rather than a separate score screen.
Jax has the clearest known immediate failure gates. Zooble has an officially confirmed Pool Scene choice that changes a CG, but an album variant is not automatically a new ending. Keep an ending checklist and a gallery checklist separately so one missing image does not make you replay an already completed finale.
Vocastar has also stated that no additional endings are currently planned for the existing characters. Caine and Kinger are being developed as new routes, so their future content should be tracked as route expansion rather than assumed secret endings for the current five.