If anyone has any suggestions for e-ink readers that are not vendor locked, I'd be interested in seeing them.
You might like the Kobo ebook readers. I have two, and I only had to connect each one to the internet *once* to activate it, and have never connected them since. I side-load my epubs using Calibre (free software). Calibre can also convert between ebook formats, and there are plugins that can make DRM not be a problem.
One of my devices has a micro-SD slot, and the other has 32 GB built-in. If I'm using them regularly, I have to charge every month or two. If they're off, maybe every 6 months.