The Dasung's Paperlike monitors are a bit expensive. Onyx Boox Max2 [0] seems a bit more recent, slightly larger, has many more features, including usage as an exterior monitor through HDMI. It's an android-based ereader, with extensive note-taking, drawing, pen writing, music sheet drawing features. It's also cheaper $500+, compared to Paperlike's $900+. I'm not a fan of bloat, which is where Max2 might be leaning, judging by a quick glance, but I'm enticed by it doubling as a large-format (portable) ereader.

[0] https://www.youtube.com/watch?v=fdRFMWff5CI

Wow a HDMI input is so great, I've been looking for a tablet with something like that in order to make it double as extra monitor on the go. Tried duet with the iPad but it isn't good enough over lightning with video compression and such.

I was doing this with a Nook HD+ (still a pretty good deal second hand as a 9 inch HD screen if you see one cheap) and TwomonUSB a few years ago. Not the most stable software in the world but it worked well enough as a second screen (hot reloading webpage and terminal stuff on it, code on my main screen)

I really want some e-ink screen setup for a RPi but not sure how it will work.

You can get off the shelf for that easily, but the large modules are $450+$350 (https://shopkits.eink.com/product/concerto-driving-board) and not ideal for a mobile application.