I think the major ones are actually just android. If you were to jailbreak them I don't see why you couldn't just use any app then.

I am very happy with my Pocketbook. Can easily install koreader (an ebook reader app) and connecting to a calibre server on my local network works very well.

selfhosted ebook library

Is that what we call hard drives, now? :P

I have two android tablets, one 7" to read small books, and one 13" to read US Letter format books, I took the cheapest ones I found, disabled Google Play and installed F-Droid to install FOSS readers, and it just works perfectly. You really don't need anything specific to just read text, you just want to make sure that you can display an entire page on your screen in a size you're comfortable reading, otherwise PDFs becomes quickly insufferable.

I used an Asus Android tablet (Android 7, it was OLD) with a giant SD card +& Moon+ Pro reader app. It syncs reading progress & bookmarks via Dropbox, WebDAV, or Google Drive. I moved to a Fire 10 that I added Google Play Services to. It can sync with my phone or any other Android device. I don't bother with calibre-web as I don't have a PC I can keep turned on 24/7 yet, so I just copy over my Calibre library to the SD card. 15k books, 512 GB SD card with ~300 GB left. Moon+ does take a bit to add new books to its database after I think 10k books.

Kobo Clara 2e