My middle son (age 11) has an anxiety diagnosis. He has some of the traits you describe: very independent (strong ideas about what he wants to do), hates school, high high and low lows (and can shift suddenly between high and low), big family-halting meltdowns, hyperbolically negative self talk, can be quite rigid at times, thoughtful, funny, creative, active, and seeks novelty and adventure. He's helpful around the house and is loving and sweet. He has always been very inattentive and spacey in school and his teachers have suggested various diagnoses since Kindergarten (autism, ADD, language problems). We've never had a full evaluation for him, but he did get tested for the gifted program a few years ago and his cognitive scores ranged from like 85th%ile to 99th%ile. On achievement tests from year to year he can go from scoring 2nd (seriously) to 95th. His brothers (and parents) are all 99.9%ile, so it's a little unusual that he scores so (relatively) low. I strongly believe that he has significantly slow processing. I don't know if this causes the anxiety or is caused by the anxiety or both or neither. It doesn't sound like that's what's going on with your son, since it sounds like his performance varied from section to section, but it's interesting how other difficulties (like slow processing and attention problems) can interact with anxiety/sensitivity.
He had counseling for a few months last year which was helpful because it gave us all some vocabulary to use when talking things through. Not a magic bullet but helpful. Our general approach to his anxiety and meltdowns is to give him space to calm down and then talk things through with him (letting him do most of the talking and being accepting). I kind of feel like he's doing most of the figuring out how to be him by himself and we're just there to support him. I do wish we could be more helpful in making life easier for him! But as long as things seem to be heading in the right direction, I wouldn't consider medication.
He just started middle school this year, a big normal public middle school, which I thought would be a disaster (he went to a small, project-based public elementary school), but actually he's doing surprisingly well. Well his grades are bad and he hates school more than ever, but he's generally happy and gaining confidence in himself. I think the more independence he has, the happier he is.
