Tip with Qt: you can use QT_QPA_PLATFORM=vnc and the Qt app will be exposed over vnc (if you have the plug-in installed / linked ofc). Pretty useful for testing embedded hardware from your desktop (not for deployment though, as it will be the only way your app will be rendered)
But thanks for the tip, I'm going to try that and see how it works. Maybe even get the old machine running the new UI via vnc.
Based on a few (possibly old) videos of the Nio I found that show the touchscreen for a couple of moments the system appears to have fairly high input latency (~1.5s), and zero realtime feedback (eg, no "pressed" state indication). Hrm.
FWIW, I've been looking around for quite some time for a general-purpose way to do easy-to-maintain UI design that isn't Gambas Basic ("how to basic?") or Electron ("what is RAM?"), and my current to-have-a-look-at is actually the drag-and-drop layout builder in the Godot game engine, which has a 2D mode, and from what I've seen of it is rather fast and makes reasonable efforts toward performance.