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.