KOreader is a great app for ebooks, and i’m pretty sure it can sync your progress with your phone as well. I’ve put syncthing on my e reader and phone so my books are synced across both devices. I’d also recommend you look into selfhosting wallabag, because you can set up koreader to fetch your saved articles and webpages as well. It’s really handy.

It’s more of an ebook manager. I haven’t used a kindle myself, so I can’t give you the best comparison there. It gives me an easy way to access my books from anywhere though since it’s essentially a website connected to my home lab. As long as your e-reader supports downloading epubs and pdfs from websites, this should be a good solution for maintaining your ebook collection