Gnome and Plasma both recognize 4K, or should. So Plasma has a builtin RDP server you can enable in settings, which connects to the console session every time. I'm not sure if I would call it rock-solid, but it's not terrible for remote work, and usually it's the shit internet connection that gives me trouble.

Using Xfreerdp client or Remmina from linux boxes seems to be as good as the RDP clients get, and I don't generally see any issue with it. If I remote into one of my boxes with the plasma RDP session enabled, I see both monitors at their running resolutions in the RDP client. I'm using Krdc as a client, but it's maybe a bit flaky as a client which is why I recommend the others.