In texas during the summer we often take showers with no hot water (or very little hot water).
I’m not sure that’s a good idea. 120F is in the danger zone for Legionnaires' disease. https://en.wikipedia.org/wiki/Legionnaires%27_disease#Preven...:
“Keep water temperature either above or below the 20–50 °C (68–122 °F) range in which the Legionella bacterium thrives.”
Of course, you might say, why not put the thermostatic mixing valve at or in the shower, which also works pretty well!
That's clearly a case of conversion results being given unwarranted precision.
A tankless water heater with a feeder pump, dedicated to bathroom use and with a bathroom-installed remote control, makes this trivial. It's probably also more efficient, easier to use, and (vs. thermostatic valves) about as easy and maybe also cheaper to implement than any fussing with valves.
A 30-year-old tank heater in a basement that needs a wrench to adjust a temperature that won't meaningfully change in a 2nd-floor bathroom for another half-hour, and also has to serve a dishwasher and clothes washer that might also run at higher temperatures at the same time, can't swing this as easily, or at all.