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)
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
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.