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.
² https://pgaskin.net/kepubify/
³ Though you don’t have to; unconverted EPUBs load fine.
⁵ https://www.theverge.com/21507390/pocketbook-color-review-e-...
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.
BTW, I purchased the discounted "sponsored" version that shows ads but employed a minor "hack" to turn off the promotional messages.
I also don't purchase any ebooks from the Kindle store (which makes those promotional messages particularly pointless and annoying) as there are plenty of legitimate sources that provide DRM-free ebooks in standards-compliant EPUB format that I can back up on my own computer in perpetuity, and transform into Kindle-compatible MOBI files on demand for transfer to the device via USB.
Recently I decided to try a Kindle Scribe and I'm now completely taken by it. There's something so much nicer about reading on an eInk screen that's approximately the size of a real printed page.
I only use the scribe for reading, though I may try using it for notes when the software improves. I don't really care about that through. In any case, I find myself reading much more than I used to. It's not water proof though, so I probably won't take it to the beach.