Calibre does the trick.
It's a shame there's no good hardware ereader to use it with. Every e-ink device is closed hardware, runs only closed software, and is trying to lock you into some DRM-rich ecosystem.
Actually there are ways to install software on many ereaders. About 10 years ago I used PRS+ on Sony's PRS-500. Nowadays Sony's ereaders are pretty much dead, but there is alternative software for Kindle, Kobo and others. By quick googling I just found:
https://github.com/koreader/koreader
https://github.com/ccoffing/OcherBook
https://github.com/lgeek/okreader
Also:
https://onyxboox.com/ - this is Android so I guess it's bit more hackable
https://the-digital-reader.com/2015/06/19/is-the-booq-cervan...