A Kobo ereader is as open as I need it with koreader installed. The Kobo Libra 2 black and white is the best ereader I have ever had. (not the color version,,unfortunately a step back in reading quality for normal reading)
These are not competing with Kobo. These are micro-ereaders, often much smaller than a phone. They're characterized by extremely weak processors and minimal hardware. 'Apps' are not a concept here yet.
remarkable / remarkable2 also work with KOReader. KOReader provides a very good ereader experience - can import dictionaries, export word lists (ie it tracks words you look up), and supports numerous ebook formats and lets you modify the padding/margins/layout. A very good piece of open source software
It is very good except the UI could really use some love IMO. I use it on all my kindles but it really took some doing to get there in terms of getting comfortable with using it.
I’m scared of installing koreader on my kobo. Right now my kobo syncs to my calibre web server seamlessly. Does koreader have the ability to sync your library to a calibre web instance?
On mine, koreader is an app that you launch, and the stock interface seems to unload completely until you exit koreader. Then the whole "OS" is still there untouched and you can get books from hell dimensions like Overdrive.
Despite never using the stock interface anymore I've left it alone because it's not much of a bother to relaunch koreader or Plato on the rare occasions that I have to.
I got the colour after being on an old paperwhite for many years so thankfully it's a big step up for me.