Isn't there a physical limit to solving this problem? For me the problem is typically, that the water in the hot pipe has cooled down, so the first few seconds/half a minute it slowly rises in temperature until finally the pump of the boiler sets in to pump actually hot water back into the system. Only then can I reliably find a temperature mix that stays stable.
Quite a bit more expensive than a regular thermostatic mixer shower, but since I was having the whole bathroom done I figured why not spend a bit extra on some nicer stuff. The labour is still the most expensive part by far.