I can't believe this is the direction they're going. So Euphie really just became immortal? The solution the ENTIRE TIME is for these two IDIOTS to just get FUCKING MARRIED and rule together.
The solution
wasn't to "just get...married and rule together", because they're both women, and the nobility would never allow for a royal partnership that didn't secure an heir. That, and they all despise Anis, who has never had support among them.
And, the nobility are too powerful within that kingdom with respect to influence on the Crown, and any attempt on the part of Euphie, Anis, or Anis' family to "unilaterally" install them on the throne would almost certainly lead to forceful backlash, if not outright civil war (and the nobility have a monopoly on magical force in this kingdom, and even pre-contract Euphie wouldn't stand a chance).
Beyond that - the nuance of their entire fight, is that Euphie is also now inhuman, and the spirit they're talking to in this chapter was once human who did the thing Euphie's doing now, and
she eventually pulled back from ruling and reinstalled "human monarchs" because she become so detached and uncaring about 'short-lived mortals' that she risked more harm than good for the human nation under her care.
Anis knows this, and didn't want that to happen to Euphie.
And, Anis saw it as her duty--maybe her penance, perhaps, depending--to become queen alongside some noble-picked dude, have babies, and relegate herself to being a shell of a monarch because her brother was exiled and she feels responsible and owes it to her parents and nation to "play her role".
Which
Euphie took direct objection to, and thus they had their giant fight over who would succeed Anis' parents.
Euphie won (of course), and they're now together after Anis calmed down enough to realize that this might be for the best, but at present the issue of Euphie being 'no-longer-human' exists, and over time could become a real problem. Sure, she can now basically ascend to the Crown with the blessing of the Spirit Religion Obsessed nobility and claim literal founding precedent as her right, with the official adoption by Anis' parents only strengthening that claim. This shuts up a lot of the nobility that would otherwise object, and because Euphie is immortal, she could rule for...well, forever, potentially, making the "heir issue" moot.
But this still leaves Anis rudderless, which is what Tilty brings up in this chapter about "spreading Magitech far and wide" and using that in conjunction with the sheer force of power Euphie represents to actually achieve what she envisions. This is a boon for Anis, obviously, and wouldn't have been possible if she'd gotten her way - she'd have given up on
everything regarding her identity or goals if she'd beaten Euphie, but again - she was focused on "responsibility" at the time, and not wanting the girl she loved to lose her humanity.
They still have to deal with the fact that one of them is immortal and will eventually forget the other ever existed, along with her own "human emotions", but that's also why this is like maybe halfway through the LN series' plot (roughly, not 100% on that).
TL;DR there's actual reasons "get married" wouldn't work before, and also why Anis was so against all of this. It's working out for the best, but problems still remain that require addressing, even if they're on the personal level more than they are political/macro level.