Kobos are¹ good. You can load them with your own files and they support a nice range of formats. Before loading EPUBs, convert them² for better performance³. Old models keep getting software updates for years, and those can improve performance.
PocketBooks⁴ look interesting, especially in the colour e-ink offers⁵ but I can’t personally attest to that.
Take a look at the Good e-Reader website⁶ for reviews of several brands and models.
But Kobo is quite impressive & flexible. Great out of box & hackable ecosystem. Prices are reasonable, the base software is pretty great, and as you say, software updates just keep coming. I was running an ancient but lovely Kobo Aura HD until a year or so back, a fairly ancient device, & was still getting updates, getting faster/snappier & ever improving navigation/UI. My newer cheaper device has been great. Kobo has been such an excellent company to their users; it's been, ah, "remarkable" to see.
The Kobo also has a fantastic aftermarket world of open source software. https://github.com/topics/kobo?o=desc&s=updated
For a while folks were starting to get their own regular distros running on Kobos too but I'm not seeing much follow-through.
The reMarkable probably has an edge in overall open source tooling. They definitely have a much better page showing off what's available, https://github.com/reHackable/awesome-remarkable.
But Kobo is quite impressive & flexible. Great out of box & hackable ecosystem. Prices are reasonable, the base software is pretty great, and as you say, software updates just keep coming. I was running an ancient but lovely Kobo Aura HD until a year or so back, a fairly ancient device, & was still getting updates, getting faster/snappier & ever improving navigation/UI. My newer cheaper device has been great. Kobo has been such an excellent company to their users; it's been, ah, "remarkable" to see.