Technology Connections (amazing Youtube channel btw!) has a few videos on using an Onyx Boox Max as a 2nd monitor:

https://www.youtube.com/watch?v=ytpRnRke6I0 (Intro)

https://www.youtube.com/watch?v=7NfX0vlCa4k (2nd channel longer look, mini review)

https://www.youtube.com/watch?v=D38dcArwCNc (6 months later, tips / tweaks / long term review)