This manga is legitimately incredible, the author is absolutely nailing her unique premise and the character dynamics are both believable and extremely compelling. I love pretty much everything about it.
To add my stick to the Erika discussion: are we sure her motives are even that selfish? Whenever a character constantly claims in internal dialogue they're totally self-interested, I get suspicious (see one Sorawo Kamikoshi). While Erika might tell herself that she only cares about cheering up Koto, she has been unfailingly helpful and kind to Aya ever since the latter reappeared; she hasn't even really had a negative word to say about her, even in internal narration. As much as Aya's disappearance caused Koto to remain stuck in place for 7 years, the same thing I think happened to Erika; she's been chasing Aya's shadow this entire time, and only after meeting her again did Erika really internalize Aya was just an ordinary girl like her. Her focus on Koto masks it but I think Aya's reappearance is causing her world to shift just like it's making Koto's world shift.
Anyway though Aya is my personal favorite character right now. While she may put on the face of the bold, confident girl she used to be, in reality she is a complete mess who is (very understandably!) overwhelmed by the massive struggles she's facing and is barely holding herself together. The past her wanted to escape, and she got her wish, but as a result she's completely lost her place in the world and any obvious way to gain meaning in her life. Due to her anomalous state as a 14-year-old who is legally 21 without living relatives, she is completely dependent on Erika and Koto, neither of whom are really considering what's in her best interest (Koto due to her massive and still unaddressed trauma from Aya's disappearance, Erika because she's still prioritizing Koto and also likely is still unconsciously putting Aya on a pedestal). Hopefully she can find some real support at her new part-time job, but realistically I think the only person with both the knowledge and emotional capability to help Aya find a meaningful life path again is Erika -- but Erika would have to give up on Koto in order to essentially be Aya's surrogate mother, and that's not something she's remotely willing to do at the present time.
It's such an interesting confluence of realistically flawed characters that feels like it can only be headed for disaster, but I don't get a tragic feel from this manga, more bittersweet leaning hopeful if anything. I have no idea what the romantic endgame will be, I kinda feel like we're heading toward none of the three protagonists ending up with each other, but I think they'll all find their own happiness by the end. It'll just be a long struggle to get there.