Hm... I get what you mean by if it's done right... but ngl... the sheer number of bad romance, inclusive in the romance genre, is so big... they're written so badly the bar is incredibly low... to the point this one is even on the good side for me... ╮(╥﹏╥)╭
I know they seem to have little to no chemistry and he does kinda treat her like a lil sister most of time... but try to see the pros of it... 1, as a regressor it would be weird if there was chemistry with someone before they hit adulthood... 2, at least she's been part of his life more than other women and he let her be beside him when he was in vulnerable moments which he wouldn't let the classmate be if it was her...3, I do believe he cared deeply about her but not in a romantic way... at least until she confessed... it may seem abrupt but honestly it doesn't feel unnatural because it does happen... someone confessing can make you start to see them in another light and see them as potential partner instead of just someone you are close with. 4, at least... she is normal... not a stalker, a friend that acts jealous like a girlfriend or someone that keeps pushing themselves unto mc at every opportunity... like, can you imagine how much worse it could be if she was one of those bad romance stereotyped fl? An arrogant tsundere, a obsessed yandere, a girl that needs help all the time...
So... to me it's not the best... but it could be so much worse that I'm just glad it's not... u.u'
I'm not saying that this is the worst romance ever. I'm saying its just done in a rushed way. I would have liked it if she went abroad and came back and then confessed, with that she would have been sure of her feelings and developed as a character. As for your first point she is still clearly not an adult. At least not in mind, her character is still very childish.
I do agree it could've been developed better... maybe shown more of his feelings changing from seeing her as a younger sister/close friend to a possible romantic interest/someone he liked. I don't think they needed to give her more time because she clearly had feelings for him since long ago... and even with how little she appeared and demonstrated her feelings there were still comments like "this chick again?", "pls no romance" and she getting hate just from existing as a female close to mc...
I do disagree about age tho...she's clearly 20+ no? Not sure the exact ages but definitely adults... and besides confessing to him kinda on a whim before leaving I don't think her actions were really childish... even her bursting into the conference is very understandable... even more so since they were already together at that point... personally I think she feels more childish than normal cuz mc acts way too mature for his physical age... but yeah... I get your point... I'd have liked to see a bit more of their relationship instead of a timeskip...
For one I have no problem with the romance, if its done right. Which is not the case here, it came out of nowhere, I swear to god he had more chemistry with the highschool class president or with the uni classmate who was the professors daughter, He had more "romantic" interactions with them than with the female lead. He always treated her like he does his sister. The part where he was worrying about her partying and stuff in America was also stupid because clearly it just looks like an older brother worrying about sending his litle sister to study abroad. It feels like a rushed endind with the "Lets give him a love interest so he has a normal life after all its his life story" plastered all over it.