I don't think that Noah should be totally uninterested in Rachel since I think the end goal of this story is for the two of them to get together. But he's just too interested in Rachel too suddenly, I feel. Like the first chapter has him shooing her out of the room and then, 2 chapters later, he's already prioritizing her over his rude friends while being really afraid of offending her. I just want to see more of the build up.
That would be pretty interesting to see happen.
Oh totally, this whole series is predicated on the both of them making really unsound decisions. But that's what we're here for, isn't it?
IDK if using chapters as a basis to how fast their relationship develop because that scene where he was shooing Rachel away is from a few years ago and it's only supposed to be 3 volume manga. And even back when they were kids, Noah protected her from the teasing when she first moved, that's part of why Rachel was even in love with him in the first place.
I would say the build up happened with Rachel's constant visits to Alicia. If Rachel was able to fall in love with Alicia (not romantically but ykwim), during that same time frame it's not far-fetched that Noah's feelings was changing parallel to the feelings Rachel felt (seeing Rachel be the source of Alicia's happiness, like how Alicia is the source of Noah's happiness but eventually loving them for themselves). Seeing Rachel love Alicia so much definitely puts her into a new light to him. But we mostly don't see it because we follow Rachel POV.
We got a hint of it during his POV Chapter tho. His subconscious feelings only started giving him turmoil after he realizes Rachel is of marriageable age and already has a potential husband from jealousy. With how he reacted with Alicia AND Rachel moving away, the author already put hints before the marriage agreement started about his feelings for Rachel. So now he's a bundle of confused feelings especially after Rachel suggested their arrangement. And it doesn't help that Rachel keeps saying he's like a brother, when he is already automatic brother-zoned by his first love.