I've looked into this in the past and settled on Kobo. You can disable the telemetry and never use the the Rakuten account part and have a very good ereader... And you can install the open source KOReader software.
https://github.com/koreader/koreader
MobileRead forums and wiki are a good resource for ebook stuff.
For example, a breakdown of the hidden configs on Kobo devices https://wiki.mobileread.com/wiki/Kobo_Configuration_Options