Negotiating for less HW is not possible--she is at a GT magnet and the HW load is kind of...part of the school identity? I know, I know. Anyway, consider that one totally off the table.
Regarding challenge level--she is not extremely underchallenged and she is certainly not out of her depth. It's okay. I would like to see better assignments, but so would everyone, right?
"Work periods set with a timer and breaks in between"--this is a perfect, great idea. I actually do think this would make it take less time.
I also like the plan idea. Maybe taking 5 minutes to make a plan would help a lot. She likes to do things like this.
Regarding a dx, DD is super frustrating in that she shows signs of something being off but is very high functioning and also performs well when assessed. We are tired of paying people and/or asking people to assess her and coming up with nothing. At this point, I just try to cope with issues as I see them.