I don't think the relationship development was rushed but I would really like to see more from here... I would particularly like to see more exploration of how Akira feels about the polyamory and how she deals with her jealousy issues. But I think there's enough to read between the lines - interestingly I feel like it's okay that Akira doesn't show strong feelings for Mayuki outside of their triad (other than appreciating how good a kisser she is lol), because it's the context of them both loving Rin that makes Akira love and appreciate Mayuki more. And Mayuki comes to a deeper appreciation of Akira than her original senpai crush as she sees how far Akira will go for Rin. The triangle is actually stronger for this I think, because they're not just invested in each other as three pairings but also each in the other's relationship with their third partner, though again it'd be nice to see Akira adjusting as the one for whom this didn't come as naturally. Luckily Mayuki is definitely very invested in Akira/Rin and will make sure they have plenty of time together!

And I definitely don't think Rin reciprocating Akira's feelings came out of nowhere, she had only been thinking of her as a precious friend but given her polyamorous proclivities it makes sense that that friendship-love could easily become romantic love. Sometimes you don't have a full crush on someone but they confess and you're like "I do already love you so let's give this a shot!" She says she didn't reject the confession out of a lack of interest, after all, but because she was afraid of hurting Akira and Mayuki by choosing one or the other.

That is my meta anyway. I would love to see at least some short continuations, maybe with them being older... also Akira and Rin were the only pair that didn't get to kiss with tongue and I think that should be rectified!!