Unfortunately, the 18+ scenes don't impact the storyline in any way, so I haven't given them much focus or attention. As such, the only way to unlock all of the spicy scenes is by sleeping with Ren in Day 1.
If it's not too much of a hindrance, you can always sleep with Ren during day 1, fast-skip and pretend it never happened, and sleep with him again once you're ready! Ren won't ever explicitly bring it up afterwards, so I don't think it'd be too immersion-breaking.
But if I ever come back to the 18+ scenes, I can look into adding an additional checkpoint during Day 7 or something^^ Thank you for your feedback!