I have an Onyx Boox tablet and use ubooquity as an ebook OPDS server on my unraid box at home, it has an online reader that’s pretty good, but I just download the ebook file to local storage and use the much better reader built into the system. I’m a slow reader so I dont have to do it often.
I haven’t really found a third party reader that is e-ink optimised and can seamlessly integrate an OPDS server. I’d like to find one, particularly if it has syncing between devices as I also use a foldy phone as my main device so it seems some use as a reader sometimes.
I also self host a huge archive of manga in Komga, and access that on the tablet and phone via a tachiyomi fork, it handles e-ink optimisation pretty well. It also doesn’t sync between devices but if I use the komga web reader it does, it’s just a bit power hungry on the Boox and has no offline functionality so I just manually keep in sync which isn’t that hard.