We have download support and "To Read" might be a collection or a readlist, so you could just download those series onto your device of choice if you don't want

We have download support and "To Read" might be a collection or a readlist, so you could just download those series onto your device of choice if you don't want to user our web reader. If you use the web reader, of course, progress is tracked for you. If you're talking more like how Plex does sync, I literally just tried that with a Progressive Web App last week, but it didn't pan out. There wasn't enough control with a PWA (which means I'm going to have to write something native-ish in the future).