There is only one game I can think of that did totally different endings for different girls and even combinations of girls and it is a MUCH shorter game that Eternum. (Not gonna name it here so don't ask for it)
If you're talking about the game I think you are, I'd like to add that doing those endings took 18 months, over 7,000 renders, and over 300 animations. It truly was a massive amount of work. And they definitely are not doing that many endings for their next game.
Considering our average Eternum update is about 1,200 to 1,600 renders, I agree there's no way we're getting endings with that many full variations unless we want to wait years for it.
...and it's things like that that make me think that at least some of it is "gameplay" as opposed to "you don't have to be with a LI you don't like." Some of the decisions are fairly obvious--if you don't talk with Annie after her first scene, that closes off her route. But Dalia's first off-ramp is to not peep on her in the bathroom. Peeping on her kicking her off her route I get; not peeping, not so much.
Choices like this feel a bit like artifacts from earlier AVNs and Japanese dating VNs where tiny, sometimes unintuitive choices can completely derail you for "replayability". Personally, I can't be arse'd to deal with that and just use a walkthrough for AVNs like that. Thankfully, these types of "gotcha" choices are much less common in newer AVNs, I've found.