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.
Yup. That's our situation, too. As you know, we haven't had any of ours formally tested, but I'm pretty sure I know what a neuropsych would come back saying. These subclinical kids can be like that. On the plus side, sometimes getting a Dx becomes a kind of stigma to them, and the absence of one allows it just to be a learning or temperamental style. I find that it has helped to explain their own learning profiles to them, based on my observations, including examples.
With ours, one of the things that has helped has been writing one's own homework (or homeschool) schedule, initially with guidance, including breaks for snacks, playing with sibs, moving around, etc. It's an opportunity to see what needs to be done, how long each one will take, how much time you need to allow for everything, and to see each one being checked off the schedule/list as it is completed.
We also did use the timer during that 10-yo year. 10 minutes of homework at a time, with the kitchen timer counting down in front of you. I usually specified a certain amount of work to be done in that time, as there is research that ADHD-type students actually perform better when under time pressure. ("You have 10 minutes to finish the next 5 algebra problems.")
Oh, and shameless bribery (aka, positive reinforcement), in the form of very small food (candy?) rewards after timed intervals of continuous work...in the classical ABA style. They are not too old to work for M&Ms.
