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!!
This is a great analysis, and for the most part I think you're spot-on. There's some things I do disagree with though.
An exploration of Akira's feelings is, to me, not just something I'd have liked to see. Rather, it's the missing piece that was left out of the story. It was a key aspect that needed to be addressed, and while one could say it was there if you read between the lines, it's too important to the story to be be left ambiguous.
The driving force behind the story's conflict is the pain of Rin's past experiences with others' refusal to accept her polyamory. She did everything she could to keep herself from falling in love because she never encountered somebody who'd been okay with her loving more than one person. That's why, when she learns that Mayuki might be similar to her, she lowers her defenses and opens up, eventually falling for Mayuki entirely.
Akira, on the other hand, vehemently opposed polyamory, right up until the point where she just... didn't.
Rin accepting Akira would be fine if Akira had accepted Rin, but reciprocating Akira's feelings should be impossible when she was behaving exactly the same way as all the friends that ended up hurting Rin in the past. Akira never went through any growth that would have lead to her accepting the fact that Rin would never be able to choose her and her alone.
Frankly, if you look at how many people say they didn't like Akira, even after the story ended, it's fairly plain to see that she was heavily shortchanged in the character-development department.
It's such a shame, too. Canno had done a fine job with developing Rin and Mayuki, I was eager to see Akira earn her place by Rin's side. I'm certain she could have had a great story of her own, but she never got the time to have it.