There are cheaper options, but I love my phone sized Boox Palma 2. It runs android on a midrange spec chip, so you can do pretty much whatever you want with it, including Libby and old libraries that might be locked to an app.

If you want to go smaller and the other direction on functionality, the Xteink X4 is cheap and really interesting. It runs on a hackable esp32 and will magsafe to the back of your phone.

Supernote has two sizes, I use the smaller one as an e reader successfully. About the height and width of a largish novel, but much thinner, and it runs a very open android fork that lets you treat it as a USB, along with various cloud sync options including self hosting. It's primarily geared towards writing, but it natively supports kindle app for library books and is pretty easy to side load other apps onto. My one big caveat is that it isn't backlit which makes night reading a pain, but some random person on reddit just started selling a light that clips directly into the pogo pins on that back and it works pretty well, though it's annoying that it isn't directly integrated so it's another thing to keep track of

Kobo. Simply upload via calibre or another ebook management software, basically drag n drop. I use a Kobo clara bw. It can be a little slower than I want at times, but still very much gets the job done.

I still have an old Kobo h20 laying around not being used since I got the updated version. If you are situated in the eu, we can look into a sale! The h20 works fine to this date and works like a flashdrive

I recently got a Kobo Clara Colour and I've been quite pleased so far. It was trivial to upload all my books from Calibre onto it. Not the cheapest, but the resolution is sharp and you can actually turn off the backlight unlike some others.

My wife has been rocking a Kobo h2o for about a decade.
You can just mount it as a usb stick

I’m waiting on a Kobo Libra Colour right now. I looked at it and Boox. I went with Kobo because I didn’t want an Android tablet. If I was willing to spend more, I would have considered the Pocketbook which runs a custom Linux distro or the Supernote which does run Android but is super repairable and upgradable.

TBH I think an old Kobo or Kindle with some custom firmware can allow you to load books onto them easily. Also, On Ebay you can probaly find an older model for pretty cheap or find one refurb if you want to pay a little bit more.

I love my little XteInx x4. Hides in my pocket. Great for epubs from my friend Anna.

I have an old pocketbook. Works awesome