So far, Kobos are the way better option in my opinion. No ads, and it's much easier to add your own books. It's (currently) a much more open system. But, not without fault. They've shut down some older readers for no good reason.

"Easier to add your own books"... it depends. Yes, if you have ePubs and want to transfer files to the reader via USB, Kobo is marginally easier. But Kindle is easier for wireless delivery (regardless of format), and supports it on all of their models instead of just a limited subset.

I have KOReader on my Kobo device, with a couple taps I can connect to my desktop instance of Calibre and transfer books in a flash.

https://koreader.rocks

There are some awesome independent tools to get files on the device via the web.

Try https://send.djazz.se/

But you’re right. Via email is easy. And I’m mostly thinking of epubs/mobi — but drm free.

The Onyx Boox readers have a feature called BooxDrop which runs a web server on the device when you enable it that provides file management and upload. It simple, wireless, and works great.

The readers work perfectly fine without an account and the Poke 5 I have is a fair bit smaller than the last Kindle I had with the same size screen.

It runs Android and I also use Termux plus a bluetooth keyboard with it for a rather nice minimal writing experience.

I've been tempted by their products in the past, but unless they've improved their stance on their use of GPL covered code in a non-compliant manner I don't want to support their business.

It's a fair point and since hearing about that I've been in two minds as to whether I could buy another device from them.

They also use incredibly annoying DRM

If you put them into the Kindle Kids mode you get a much cleaner, more streamlined, ad-free experience without paying extra. I've seen a few adults say that they prefer it to the full-featured mode.