my wife has a Kobo reader and it’s a great alternative, from Canada. The reader works great with Calibre on desktop for books you already own, and the Kobo store is more or less equivalent to the Kindle store.

I have no suggestion for getting files off an iPhone, but presumably an app exists to arbitrarily send files to desktop, and from there Calibre works.

Kobo build quality is better than other e-readers, and it supports color and markups. Overall it’s pretty good for PDFs/textbooks and novels, but manga/comics can be a little goofy.

I cant speak on the syncing since she has only the one device.

Good luck!

Edit: seems like you edited (or i misunderstood) the OP. Kobo (the device) works great with US library lending, but ymmv if you are in another country. If you use the kobo app on your phone it will sync your position with the device, but the app is pretty flawed on mobile and doesnt have a desktop version i’m aware of.

I wouldnt mind using the app to read fiction, but it’s not great for reference material. I use a standalone pdf reader for that kind of thing on my phone, which obviously doesnt sync.