I don't have much to add, you have a point and I already had an idea, I certainly said a system of achievements would increase Cari's work even more, and then it depends on him, in the end we see the story he wants to tell and appreciate it, it's just strange to feel that only I'm not losing anything so I can start taking all the "certainty" choices and moving forward even though I prefer to play "blind" if I do that it would be so penalized losing content, and that's why I can only leave it here, but certainly everything you said is valid and I just want to be able to help directly or indirectly as possible, wanting to support his patreon in the future. But as you said, it's a double-edged sword, where I just feel that I should follow the straight line drawn as it was initially thought by the dev, and going out of it will be penalized even though I would skip a charlote route with pleasure XD.The "one route is the true route" hearkens back to the old-school way of writing VNs and was present in most eroges that aren't your average "school setting with more than a dozen love interests or so and no actual plot except getting laid." Branching paths are pretty rare, with some only "branching" after you've done one route, with the rest being treated as unlocks that pad the VN's overall runtime (e.g. Fate/Stay Night's and Full metal Daemon Muramasa's route structure).
The trouble with branching paths IMO is that for a story that basically spans multiple hours of reading and watching cutscenes play out, having branching paths only adds a lot to the dev time required to make said paths feel "meaningful." It's why AVNs like BaDIK basically take a year per update with only an hour or so of gameplay per route (mayhaps even shorter) because the dev has to take into account the multiple routes that exist in that game as well as the "other stuff" that he likes to add because of personal preference...
/eyetwitches at he remembers DPC's unrestrained fetish of adding minigames to his work
...anyhow, the thing with Cari is that he obviously has a story to tell and he wants to tell that with the idea that all members of the harem have something to contribute to the plot and not just act as love interests that the protagonist can choose to screw. With OIAL he had only succeeded in pulling this off with Lauren and Judie, with certain members of the harem having some contribution to the plot but ultimately fell off when they basically served their purpose...and with some not even contributing anything at all and their inclusion is just because they happen to exist (i.e. Jasmine, Iris).
So with Eternum Cari is basically rectifying this issue by making all of the harem members contribute to the plot for the long haul and that itself is a double-edged sword as you have seen, because if you DON'T play it as intended by the dev you're basically locking yourself out on huge chunks of the narrative by going with a one-girl route as each of the girl's route has some contribution to the overall plot. This isn't like in most modern AVNs where branching paths or choices actually matter. On the other hand, doing it this way allows Cari to basically write meatier updates while balancing everyone's desire for lewds by making at least half of the girls have some scenes per update (and this will increase as time goes on, especially once we start hitting the multiple girls per scene segment of the story).
So at the end of the day, this is basically just Cari knowing what story he wants to write, understanding the shortcomings of going with writing said narrative, but ultimately sticking to his guns as the overall goal is to make the story he wants to make with only him doing most of the legwork and doing so in a timely manner and without having to spend more time than necessary having to write multiple paths and taking into account what choices the player made and the potential consequences for said choices. Granted, there ARE choices that the player obviously has to make that obviously would lead to different outcomes (i.e. who lives and who dies in the Collector whodunit, whose side you chose in the latter half of the Andromeda event), but they likely won't completely affect the overall narrative that Cari wants to make. And that is fine given the type of narrative that Eternum has (which is a long-format narrative and not a choice-and-consequences style of story).
And with that I'm not saying that any of them are bad so far they are all wonderful and well developed although everyone has their favorites. And in the end, you won't be losing anything, just trying to get everything possible, without being afraid and that takes away a certain weight and I just feel like I'm actually reading a story in a straight line, even if at least like in the game of detective, you can have a certain agency, it's a way around this, and I wouldn't agree with having to end route X to release others.
As I said at the beginning, everything you pointed out is in fact valid, and all that remains is to wait and support and help as possible to deliver the best product as Cari wishes it to be.