Unstable car over 65mph

But isn't that true of virtually all cars, especially far eastern - and it isn't a mass issue with every new car.

But isn't a pre-shipping check to over-inflate the tyres to minimise the risk of flat spots? Perhaps that gets missed on some cars.

It's a fair point. Geometry set-up probably likely to be one of the biggest factors based on OPs original post so I would be booking the car in for a proper 4-wheel alignment as a starting point.

True, flat spots, 4 wheel alignment, good things to do first.

OE tyres though, they are f’ing shit

Were you using cruise control? It could be that you had it in TJA mode instead of ACC so it would be trying to steer for you as well as maintain the speed. I doubt it’s that as you’d have felt the steering wheel moving on its own but it would otherwise do as you described. With lane assist off TJA will still operate if turned on.