I've often wondered if this is something that could be engineered correctly or is it genuinely a difficult problem to solve with multiple variables (incoming water temps, pressure, etc)
A thermostatic mixer is the usual solution. Set your desired temperature and the valve dynamically adjusts the hot and cold flows to produce that output regardless of input temperatures and presures.
Works great until it jams at the "instantly vaporize target" setting. Which reminds me, I must call a plumber...
In Japan, we usually have a thermostat to set the max water temperature and most bath fixtures have temperatures written on them (with a little push button safety thing to go over 40c). I don't know why it's not common elsewhere.
Was thinking on this tonight! There should be a mechanical solution. No electronics or other complications to fuck up down the road.
We're all different with comfort levels, hot water temps, flow rates, all that. We need something with sliding or rotating valves that lets us dial in a range we're comfortable with.
Set X as the lowest temp and Y as the highest. Now when you get in the shower you can spin the dial, all the way around, yet remain in your personal presets.