I really like your interpretation of Tsukishima on Yamaguchi. I want to genuinely believe that Tsukishima cares about him and it's just his issues and their fucked up dynamics that make him come across as uncaring.
To be honest I don’t think I’d call their dynamic as ‘fucked up’. It’s definitely not a perfect one, but still fucked up too big of a word for that.
I think Tsukishima is generally not the kind of person who even CAN show that he cares. Most definitely not in the way of ‘sugawara-senpai-level’, simply because such a behaviour is probably way too overhelming for him and not something he can pull off; and not even on ‘kageyama-level’, because that’s just stupid, for him at least.
Tsukishima is distant, and besides his mocking remarks I don’t think it’s that easy for him to actually communicate with people. And I guess being around Yamaguchi, is as comfortable for Tsukishima as he can possibly be in relationship. Even though it seems so one sided.
If he didn’t care in a slightest, he wouldn’t let Yamaguchi be around him. Kei is not someone who’d take someone’s bullshit on a daily basis with no consequences.
Either way, I will just lead you to a post that explained it all so much better than I ever will.