Any engineering going towards doing this on X11 is wasted at this point and waiting on the Wayland community to agree on and implement a common interface is going to take years, so a kernel-level solution seems like the only way to have this right now. FWIW, that is how SpaceDesk does it on Windows and their performance is great, so it's not like it's impossible to make it work.
I've actually already managed to get a kernel-level virtual display working, but haven't tested with Deskreen yet as it's 1am and I've been staring at C code for too long now. Might post it on HN if I end up actually packaging it up into something usable.