Ye I also really don't get why they're holding back since his friend is dating his older sister so.. so why sena is waiting is beyond me.
Because Sena is a different person than Occhi and has a different backstory and character mindset.
One of her bigger hangups from the beginning was that she felt she was "wasted" on Nao due to her lack of self-worth born of the life she'd lead since she was forced to "grow up" at age 8. Yes, she's come around on the fact that he genuinely likes and cares for and about her, but she still defaults at times to seeing him as "Occhi's little brother".
And just because Occhi has gotten over Nao's friend being younger than her, doesn't mean Sena is obligated to do the same. The tension between them still works, and it has been less than 25 chapters--the fact that some readers feel like this is being stalled out when they've actually made considerable progress
as people in their own right feels like simple impatience. We even got the time check of "2 months until Nao's birthday" recently, so that puts us around Christmas time, when it was "half a year at least" less than 10 chapters ago. They've both been working toward figuring out that they truly do care about one another and want to date, and they both have things going on in their lives that also require attention from the narrative. It's all progressing much faster than it could, honestly.
Plus, the fact the TL is working overtime to dump this many chapters in such quick succession is also a boon on this front, because it's not like we had to wait a month in between each installment where 21 chapters would take something like 2 years IRL to reach this point.
Oh man didn't even think about how most likely some new dude will show up from school... please God no...
I actually would be surprised if it went that direction, especially as Sena knows that Nao only has eyes for her, and Nao has demonstrated that he will act almost impulsively to protect Sena from would-be aggressive men. They do trust one another, and have even circumvented "misunderstanding" setups already with other characters, proving they can trust one another on some level.
My thought is that they'll just be separated physically by her schooling taking a lot of her time, meaning less actual time spent together like they've enjoyed up to now. Plus, Sena might end up feeling like she has to prioritize school, meaning she might push back their "dating deadline" even further than his 16th birthday.
The counter to the first point is that she's ostensibly still working, albeit much less than before, so they'll still see one another at work each week if Nao keeps up with the job past this point. To the second point...well, Nao is very proactive,
especially after this talk with Setsu and Kouhei helping him solidify his feelings for Sena and precisely just how much she means to him and why. So I don't see him allowing her to drift away or put things on hold when he's chomping at the bit to be her boyfriend almost more than she is at being his girlfriend.
The author's already shown that they'll work through perceived hiccups and roadblocks in short fashion, so I don't expect this to be "dragged out" in any obscene manner, and even the "usual tropes" like another romantic interest are already covered by the progress the two have made and the fact they both have a support system of friends who wouldn't let misunderstandings or problems slide.