I think one of the best examples of what went wrong with TSATS is from the book tour I attended -

At one point during the event, Mark Oshiro made a comment about Nico's card collection. Specifically, they joked that Nico collecting cards was a sign that he was gay, because clearly he was only collecting the cards to look at the men on the art (which ends up being a note made in the actual book itself).

I've said a lot that you cannot divorce PJO from neurodivergence and disability. You just can't. And I stand by that. If you remove the neurodivergence and disability aspects from PJO it is no longer PJO because that's the foundation the entire series is built upon - representing neurodiverse and disabled students and kids. If you do not understand that or try to ignore it you have missed the most fundamental aspect of PJO as a series and everything else falls apart. (This is actually a trend that begins occurring mid/late-HoO and throughout TOA and that's where I say the main series begins to feel like it's no longer itself, but that's a rant for another day.)

You cannot divorce any of the demigod PJO characters from being ADHD/dyslexic. It is a core part of their characters. You cannot separate Nico di Angelo from the fact that he is ADHD/dyslexic. If you agree with Nico being autistic-coded or not, he is explicitly ADHD, and MythoMagic as we're introduced to it with him is clearly his hyperfixation if not his special interest. It just is. MythoMagic with Nico is the main ADHD/autistic trait we see presented with him. You cannot erase that. You cannot say "Nico only collected cards because he's gay" because then you are removing the fact that Nico is ADHD and you have missed the entire point of the series. Failed step 1.

TSATS does things like this so often throughout the book. (Ex: None of the characters stim, ever. The closest we get is Will bouncing his leg in one scene, but that's heavily implied to purely be him feeling anxious in that moment and nothing else. Nico even gives up his most iconic stim object and it's replaced with a coin he explicitly never stims with. He only ever touches it, never stims with it.) The book refuses to acknowledge that Nico and Will (and Annabeth and Percy and Piper and etc etc etc) are ADHD and dyslexic (and autistic-coded, in Nico's case). And if it does even remotely acknowledge those themes, it does so in the most ableist ways possible (infantilizing Nico, blaming Nico for his own ostracization, magically healing all of Nico's problems, implying Percy is only bad at school because he's disinterested and lazy, etc). And that happens because they started on the wrong foundation. They treated the characters' neurodivergence and disabilities as secondary and optional rather than the literal foundations the entire series was built upon and it shows.