Kobo has a great balance of good hardware, good price, and good openness. It’s not perfect on any of those categories, it just strikes a nice middle ground balance to make it an extremely popular ereader for people who require the kind of openness people like us do. There’s really nothing locked down about them, they don’t do anything in particular to make it easy, but they don’t do anything to make it hard either. “koreader” installs very nicely on Kobo devices, and then you just load your books from Calibre (or right through USB if you’re hardcore for some reason) and you’re basically off to the races.
Yeah, that’s all it is. Calibre Portable. In a folder on Nextcloud.