Does the UK7700 allow the setting of “game mode” with 480p inputs? Should the input lag performance be similar to what you have reported for 1080p when using a 480p input? I’ve purchased a low latency line doubler that takes 240p/480i (e.g. from game consoles) and outputs 480p via HDMI, and I’m looking to pair it with a low input lag HDTV (ideally < 1/60 s, i.e. 16.67 ms).
rtings note in their review the 75" and 86" models don’t support HDMI VRR from reports they read.
Check that the firmware on the TV is up to date and instant game response is enabled (which is the toggle for HDMI VRR) but that’s about all you can do.