Never tried it, and I think it's impossible to do, or at least not recommended to.
The majority of the sex scenes in this game are tied to the route/ending. Forcing to "trigger" conflicting scenes might just break the save.
It's been a while since I played this game, but IIRC that once you're locked to a "route", it's impossible to trigger another route/watch their intro scenes even with leftover rests. (eg: locked into vanilla route, and you cannot trigger the NTR scenes anymore)
You can always speedrun NG+ without resting at all (surprisingly easy in 3rd> NG+ once you've maxed level with OP weapons and recovery items).
Unlock all rest spots/rest amount, progress as far before getting locked into a generic route / "no turning back point", save the game, then simply keeps sleeping (or let them sleep with each other) until you unlocked their sex scenes / route / ending. Beat the game, then reload that save. It's a timesaver.
This game was meant for multiple playthrough after all. True end requires you to play at least 3, way more if you retried halfway and doing runs snipping for lores.
I've done at least 7 full NG+ runs, from beginning to end. And that doesn't include the runs where I do the trick I just explained above to unlock that specific character(s) ending, where I just abandons that save and return back to the save where the path diverges, else double that number.
I didn't even touch any BL scenes/routes at all, so yeah, it's a very VERY long game for a completionist.
Honestly, just download the save. Unlocking everything legit might as well be impossible. There's way too many variables that can change the outcome of the ending(s).