tl;dr: if yuki annoys you so much and you're just here to see a romance develop between 2 specific characters, given might not be the story you think it is, or the one you're looking for. everyone is different and there are thousands of BL manga out there, it's ok to admit when one isn't a good fit for your tastes, and then move on from it.
Uenoyama IS insecure, and that’s understandable, but people dont even notice that he is. They just think he’s the perfect boyfriend without any issues. I think that’s what the original comment was trying to say. I dont think he’s insecure with Yuki though. I think he’s insecure because this is his first relationship and besides all the baggage that comes with it, he’s not really sure if he’s doing well. Like when he thought in Chapter 23 “to begin with, what does he think of me as a boyfriend? Also, more specifically, what does he really want to do?”
i don't know where people are pulling out this whole "perfect boyfriend" shtick, i don't see anywhere in-text that uenoyama is demanded to be a perfect boyfriend? he's a teenage boy, he's definitely not going to be perfect. and of course he's going to have completely reasonable questions and doubts about his relationship. everyone has insecurities. but i don't think that makes him insecure, i still read him as a fairly confident person. a completely healthy relationship can still be nuanced and encounter conflict, just as a confident person will still experience moments of self-doubt.
gusari has ALWAYS been about ensemble casts and writing multiple couples being interconnected through their common acquaintances, even before given. so of course yuki will be featured a lot, as he is the intersection of many characters' relationships.
this story has also always been first and foremost about MUSIC, not romance. i have never seen uenoyama and mafuyu's relationship as being the central driving force of this story, even when i first read given back when the second volume hadn't been translated yet and there hadn't been a single established side couple.
i feel like many people simply view yuki as an obstacle to the uenoyama/mafuyu romance that we're supposed to be over already, but gusari developing slowly developing his character through the lens of how all these other characters perceive him is such GOOD storytelling. yuki isn't just mafuyu's ex; he's his mom's son, he is hiiragi's friend, and he is deeply intertwined with mafuyu's experience and love of music.
(also, uenoyama Not feeling insecure in relation to yuki whether it's in regards to his relationship with mafuyu or his relationship with music is a breath of fresh air. we don't need to rehash the "im sad ur ex is better than me" trope, it's a dead horse.)