That’s incorrect, Graphene OS has Android Auto support.

If you’re ok with using your tablet as a drawing tablet with your PC, I use an app called Virtual Tablet for that. Sadly it appears to be Windows and MacOS only, but a quick search led to Weylus. I have no idea how well this works and never tried it, but from the readme it sounds pretty good.

Miracast is pretty good, but I don’t think it supports DRM-content. Also it seems to be getting phased out in favor of chromecast.