I get the mismatch- it is familiar! Gifted kid with slower absorption reading, possibly due to giftedness and possibly due to focus issues, hard to tell.
A very very similar kid’s parent wrote me back here on CC that Irlen screening and treatment was life-changing. That student was genius level but reading lagged.
The testing done by schools or by neuropsych’s does not evaluate ADHD. Primary care or a psychiatrist can do the questionnaire and you can find one online. The student and the family do one.
Not saying that is the issue, not at all.
Still, she ought to have school work during the school year that can be done without feeling overwhelmed or feeling like she has to prepare during the summer.
Just speaking from experience- burnout happens! May not apply at all and you clearly have a handle on the situation and know what is in her best interests 