7 Goodbyes helps everyone out, but I still find Nino's development notable, especially in the final chapter. Any of the quints could have almost drowned, but it's Nino. Then, there's the final panel where Nino can't let go of her feelings for Kin/Fuutarou as he bids farewell to Rena.
Vol 7 develops Ichika a bit, but Nino's feelings form a substantial undercurrent, coming to a head in 59. She tries to break things off between him and them, even trying to tell her father as much. F shows up to save the day, becomes her prince, tells her he can't accept that severing of ties, and even sort of stands up to Maruo in the process. For a supposedly Ichika-centric arc and volume, letting Nino make the first undisputed confession instead of Ichika, who was planning to do the same, is still an interesting choice.
Nino's role/development in Scrambled Eggs has been discussed enough.
Chapter 71 gives Nino a second chance to begin her pursuit of Fuutarou, and she manages to break through his defenses briefly. He also specifically wonders about her lips in a much more explicit way than he has with the other girls ("There's no way, right?").
The volume ends with her again resolving to settle things in Kyoto.
Then there's the little things. She's the only one noted to have tried to seriously assemble an outfit for him prior to the camping trip. There's also the whole piercing chapter.
And so on.
I think her development is set up in such a way that it is subtly different from the other girls. It starts off slow and small, but it becomes much bigger and more obvious over time. I think the story hinges on her growth subtly more than it does the other girls. I could be wrong.
I can buy 1-4 being the bride. I started off as a Miku fan, so I still love her.
But I find the evidence for Nino's case to be more compelling.
At the very least, I don't think Itsuki has built up a compelling case that doesn't hinge on her being first girl, which may unfortunately be enough.