Suggest you also look at the Boox Nova 2 and
https://tachiyomi.org
For example when I had a Kindle/Pocketbook, every time there was a new issue of an active manga I had to side load it and often convert it. With Tachiyomi, new chapters of series you've bookmarked are pushed to your device automatically.
My son reads manga for a few hours every day on his older Nova Pro, and is able to manage everything himself (he's 9).
I've got the Nova 2. I think 8" is a good size for manga / graphic novels. Manga is fine on a 6" too but it's too small for graphic novels.
If side loading instead of using the extensions on Tachiyomi, the built in Boox reader is very good for cbz/r.