I just want to throw it out there that while low WM is a typcial profile for ADHD, it is not always the case. My oldest DS, prior to the advent of his complicated medical issues, when his problems were just ADHD and dysgraphia, tested with a WMI of 126 and a PSI in the 90's (can't remember the exact number).

Also be aware that for ADHD kids, those IQ indexes can shift again as the kids mature depending on how well they keep up with neurotypical growths in working memory and speed.

It is possible to have beautiful handwriting prior to 3rd or 2nds grade when the writing demands are lower, especially if the kid is a good artist.