I'll give the Iceland video a go tonight and let you know how it goes.
I must be dumb or something.
After a few hours now, the Iceland video 4k60fps plays fine without any judder.
My theory is that while the TV was immature, the CPU was being used in the background doing work for Android. Now that it's not doing work it's ok.
The other thing I've changed was to enable Instant Power On, effectively putting TV in standby rather than sleep.
Perhaps this is the more critical step, perhaps the Android uses CPU for a wee while after a full boot.