This dynamic actually works. The only problem is that it took so long! Amesaka should have been part of the quest to save the world almost immediately after she was introduced. Now we know that even if she loses, her friendship with both Mito and Yumesaki will be forged in fire, and she will come out stronger and happier than she was before she met them.
Also, a proper love triangle has three sides, not two. Yumesaki sucks at keeping secrets and Amesaka actually has some brain cells to rub together, so there was never a way to let them meaningfully interact and forge a genuine connection as long as Amesaka wasn't allowed to know the secret. Now that she is, the Yumesaki/Amesaka side of the triangle can be properly developed. So while Yumesaki is still favored - there is a realistic prospect of a triangle ending, with both of them winning because they like each other too much to want either of them to lose.