I have a Kobo Libre and with KoboCloud[1] it syncs automatically with Nextcloud and other cloud providers. It’s awesome.

[1] https://github.com/fsantini/KoboCloud

I use a Kobo Libre 2 with Calibre Web to sync.

There’s this company, which makes ebook readers that don’t seem to be tied to any particular vendor:

https://shop.boox.com/collections/all

They have so many models, though, that I have no idea where to start with them.