Android-based ebook readers are pretty much popular now. I for one found writing in my Android e-ink device with Bluetooth keyboard extremely satisfying.
Joplin works in Android quite well. But two enhancements can be very helpful for us e-ink users:
Yes. A high contrast grayscale theme is what we need.
I must say, current light theme is also doable. But what is needed more is the ability to stop animations (e.g. for drawer open/close) since e-ink has low refresh-rate.