There's an app that runs natively on RM2 that supports Zlib downloads. You'll have to tweak the source to work with your personal domain.

I buy physical books and then get digital copies from Zlib (if publisher doesn't offer digital downloads that is...)

You can also just use Google Drive or SSH to copy books from another device like Kindle, Android, or a laptop.

I think calibre will do what you want. Convert to pdf because epub on remarkable2 is significantly slower. You will want to tweak your pdf conversion settings to get the font sized for easy reading.

I use calibre for managing my ebooks, syncthing for transfer and koreader to read them. I removed the DRM from buyed books, since koreader can't decrypt them.