So I decided to do a reread yesterday, to get a feel for the pacing and how the ending really slotted into everything without the delays from waiting for scanlation (thank you to the team btw), and... I don't think it was a terrible ending, it felt like the trajectory the series was sort of going, but I think giving Meguru a bit more time to fully accept her feelings and ask for forgiveness for having hurt Azuki would've been a good arc for a theoretical fourth volume, and it definitely felt like another volume was needed to properly wrap up Niji's story. I think in such a theoretical, the Niji/Remu arc would probably be the focus, especially since it really only got going in Vol3, with Azuki and Meguru finishing up their arc that started in Vol2 being sort of the "B plot". I also personally disagree about some people's ideas that Remu was developing romantic feelings for Azuki, but I also think it's an understandable position to come to. I think her worries with Azuki and Meguru making up and Azuki being "taken from her" were more a worry that Meguru would sanitize Azuki and Remu would lose the one person she felt she could confide in about her fantasies. I think that plotline actually got a decent resolution, when Azuki declared that they would be allies for life, and I think a theoretical Vol3 leading into a Vol4 would keep the points of Remu being confronted by Meguru and then confronting Niji, with a similar resolution (maybe less direct acceptance from Meguru and more of a "maybe it isn't any of business" sort of response).
Overall, I think the ending is alright, and doesn't ruin my having read this series, but I think it really needed one more volume to earn its endings.