I didn't see Xpra mentioned, but Xpra does desktop and window streaming with fast video encoding using H.265 (or H.264 and others options), and it has an experimental UDP transport - though I haven't needed it yet. In the Windows world, gamers use Parsec which matches those two network transport and encoding design decisions (which I am sure are the right direction).
To use Xpra as part of a secondary monitor solution, you might need to define a "dummy" window region for it, and I suspect X server can be configured to allow this but I don't know how yet.
I'm interested in any solutions for Linux secondary monitor devices too, especially more simple ones.
Edit: kbumsik link below uses xrandr to add a new display region and then uses x11vnc with a "clip" argument. I couldn't see such an argument for xrandr.
This would work nicely on Pinephone, because it has very efficient accelerated decoding of H.264 codec. gstreamer using kmssink consumes very little power decoding H.264 video. It would likely run at ~2W with display on, so for about 5 hours on battery (at reasonable screen brightness)
To use Xpra as part of a secondary monitor solution, you might need to define a "dummy" window region for it, and I suspect X server can be configured to allow this but I don't know how yet.
I'm interested in any solutions for Linux secondary monitor devices too, especially more simple ones.
Edit: kbumsik link below uses xrandr to add a new display region and then uses x11vnc with a "clip" argument. I couldn't see such an argument for xrandr.