Seojoon’s feelings are valid

Totally agree

I think you're kinda misinterpreting what is being said, though even so you can see however you want.
I think it's not that anyone should develop feelings after being friends for a long time, it's that SJ feels bad that JA was never attracted to him that way once during these years, cause ofc he'd have liked to be wanted. He saw JA want and pursue many things and ppl, but never him, and that's the disappointment he's expressing. Not that JA Should be into him just bc they'd spend all that time together, it was just "why not me?" feeling. That's exactly why he left, cause he couldn't handle that feeling, not because his expectations weren't met.

Ofc he should've said something sooner, but I don't think he was actively hoping for things to turn out how he wanted eventually. Wishful thinking perhaps, but mostly it was him wanting to still be near him while knowing his feelings wouldn't be reciprocated and just holding out as much as he could before he'd inevitably find it to hard to bear and leave. For his own reasons, getting over these feelings wasn't possible, so his solution is to leave without becoming a burden to JA and also without hurting his own self anymore.

And JA is actually being very mature saying he'll consider his feelings over some time, he's not making false promises nor dismissing his good friend's feelings like a inconsequential thing he can quickly reject. It's an opportunity for both to see how things can move forward, in whichever way, but just in a conscious and caring manner.

I agree with almost everything you said, and yes, he is very mature. But my point still stands. If he felt that strongly, he should’ve said something instead of expecting an outcome that was impossible at the time. Joo-an was literally searching for Hajin this whole time. It feels unfair to resent Joo-an for not having feelings when those feelings were never communicated in the first place.

I don't think he resents JA for that, he's just expressing his sadness of feeling rejected through anger on this occasion cause everything bottled up is coming out even if it's not exactly "fair". But JA was also not particularly fair since he got so used to having SJ by his side, who was attentive and supportive to every little detail of his professional and personal life, far more than any regular friend or secretary would ever do and relied on him to such a degree WITHOUT actually showing equal care and interest in SJ. Don't forget that while JA had been discussing all his romantic escapades and Hajin's stories to SJ, he admitted himself that he had never even wondered about SJ's life and interests...
All that to me show how the author wanted the narrative to be one where neither was perfect, but SJ's frustration and choice was ultimately warranted.
Not trying to convince you to change perspective, but I wanted to lay it all out cause I think it's easy to forget some context and judge without it.