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)

Even without koreader a kobo is refreshingly open to tinkering. Getting root and ssh access is just a matter of renaming a file on the disk.

I got the colour after being on an old paperwhite for many years so thankfully it's a big step up for me.

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.

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?

Yes, if you upgrade to Calibre-Web-Automated https://github.com/crocodilestick/Calibre-Web-Automated#kore...

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.