Honestly, if no poly ending I could see Kaoru and Minato developing rivals to lovers route. Mei and Maria seem endgame to me at least. Ruri seems sensitive and innocent which could be an interesting dynamic between her and Karin who seem to be opposite ends of the spectrum but I’m not sure with Karin honestly
In short if no poly I believe best couples would be: Mei x Maria, Kaoru x Minato, and possibly Riri x Karin.
Endgame's gonna be Mei and childhood friend. Then Kaoru and Minato. Then Riri and Karin. Then Maria'll have to find someone else. That, or Maria will get the childhood friend and Mei's gonna be single.