Do you happen to have ADD/ADHD as well? To my understanding the lack of or restricted internal speech can be understood as (or explained by) the deficits in the verbal working memory. Dysgraphia has a very high comorbidity with ADHD because its working memory deficit can also manifest itself in the so called graphemic buffer errors.
https://www.additudemag.com/7-executive-function-deficits-li...
https://www.researchgate.net/publication/6128763_ADHD_and_Dy...