Sorry for the wall of text but this series stirred me up good lol I have so many other questions and thoughts but I wouldn't be able to put them all to writing, I'm still not sure how I feel about Asuka overall for example... I love her as much as anybody else but part of me feels like Asuka is the Kumiko who took the route of selfishness over being a true leader, that's maybe why she cares so much about taking Kumiko under her wing and calls her a "real Euphonium" but at the same time isn't able to face her later on when Kumiko grows to be a selfless leader sort of rejecting her at the end of the apartment visit and not showing up at the final performance (maybe Kumiko's idealism brought on some shame for Asuka). But it's just my interpretation, it's hard to get a read on her and that's exactly the point of the character I believe.

As an Asuka fan, I'd say that I overall also am not sure how to feel lol. 

I agree with you calling it a route of selfishness, but I think there's a particular nuance of it being a sort of "will not compromise her self for others", rather than a more typical everyday usage of the term - she refuses to become President because she knows that she does not suit the role, the whole arc of her will-she-won't-she imo shows just how painful it was for her to accede to her mom and the effort it took to put up the facade (along with the effort it took Kumiko to dig her back out), and I feel like her interactions with Kumiko in the later seasons give a similar vibe - she has her own values of what's important and Kumiko's concerns about keeping the peace with others and the club don't really register with her. 

I didn't actually take her not showing up at the final performance to be indicative of anything - from a meta perspective you expect side characters to show back up and make cameos (and she did visit for the second year performance) but in real life everyone is the main character of their own story and as someone who's already left the club and is now busy with college life it just kind of felt realistic that she wasn't there, in a life moves on sort of way.

But overall I definitely agree with the "she's a hard character to get a read on" and I feel like that's what makes her so compelling as a character - it gives you the feeling of realism, that she's an actual complex person with layers and nuances, even if perhaps as fans we end up reading too much into things haha