Commenting to provide a confirming data point. I bought a Kobo Libra H2O a few years ago. Unboxed it, modified the config, and immediately started using it with Calibre. It has never participated in any manufacturer's "ecosystem" and has functioned well in a totally "offline" capacity.
I was sad to hear newer Kobo devices are shipping with Secure Boot. I've never reflashed my Libra H2O (it's my daughter's and I'd never be able to get it away from her long enough to replace it) but I liked knowing that I owned the device. I'm sad to hear the new ones are owner-hostile.
I used the Kobo with its ecosystem, even bought a few books with the store. Also disappointed to hear that they seemingly don't allow unlocking the bootloader on their newer devices... If that's true.
I don't care about secure boot / a locked bootloader so much as the ability to unlock it.
I'm not sure about that. The relevant discussion here is at https://news.ycombinator.com/item?id=46194337 but from the linked Github issue it's not at all clear that a true bootloader lock is involved. It should still be possible to boot something else on device, it's just likely to require a lot of work.
There is a config file on the stock OS that you just need to change, and you can point the Kobo store to your own instance of Calibre Web.
This lets you sync and download your own books to the device over wifi.
I played around with KOReader a bit but found the stock software simpler to use. All I really need is to not be tied to an ebook store.