I don't own any of these so I can't recommend, but from recent posting activity around here, a few ideas to check out:
Likebook Mimas (has the SD slot you want):
https://ereader.store/en/boyue-likeb...-mimas-43.html
Onyx Boox Note Pro (Probably a more refined device, but no SD card):
https://www.boox.com/note-pro/
Just my personal take, YMMV: I'd at least research and consider trying an Ipad Air or Ipad Pro. Or a Samsung Android Galaxy tablet in the same price range. For example, you can get the new
Air in the same price range as the Onyx ($499 for the 64GB Air, which matches the Onyx storage, is $100 cheaper, and has a faster processor and a better display). And while it won't have the SD card, the Air has capabilities, performance, and choice of apps that will blow away both of these e-ink tablets.