Caveat: I haven't purchase or used another e-reader since I bought my first one (Kobo) ~15 years ago.
My Kobo still works and the battery still lasts like a week or longer before needing a recharge.
I don't think I've run into a book file it can't read, but I mostly stick to epubs and PDFs.
I'm pretty sure it has an online store, if that's important.
For your requirements, I have no idea if a planner has been implemented in the last decade, but I would suspect Kobos can read Libby library books (not sure what they are though). I believe my ~15 year old Kobo has a sync feature, but I don't use it. So that's probably available as well. For easy on the eyes, I'm assuming brightness settings, which Kobo had and I used on occasion (they may have adaptive sensing now).
I recall Kobo used to be marked as the "open" platform, so unless something has significantly changed, Kobo shouldn't lock you in or charge you a subscription fee.
Best of luck. Hope you are able to find an e-reader that works for you.
Edit: Just found this website with a bunch of Kobos compared. It looks like some have Dropbox sync and "Dark Mode" (I assume for easier on the eyes).