Dale ; Your use of an automobile type thermostat to regulate domestic water flow is revolutionary ! By volume to be heated, and because
it's open to atmospheric pressure you have protection against '' BANG- SQUISH! However once the water has come up to temperature ether within the 1st or the 2nd tank
a very simple mechanically operated (by-metal?)control dial-in system (which is existing technology,) can be used to deliver water at any preselected temp
This is a common over-ride system in hotel/motels , this mixes with much cooler water to provide almost any safe pre-selected temperature !
This is usually a mechanically operated dial-in system that should be hidden from idiots in a locked sealed cabinet, otherwise it is still existing technology.
This is only a little more complicated than the mixing valves of a standard washing machine, mostly in the re-build.
If you have the electricity to 'run' 1 set of washing machine type mixing valves, a redundant set or sets should give you the flow and the temps. that you want !
There is a commonly used Field hot water system where you self regulate the amount of hot water you drain off of pre existing pre-heated hot water,
this works by a system where the cooler water that is added to the volume of pre-heated water, is delivered to the bottom of the container,
causing a directed /ducted over flow of hot water off of the top off the container , this overflow happens before any mixing of waters of different temps. can occur !
I hope that i have explained my points clearly
enough to be useful to this dialog ! Be Safe , Keep Warm , PYRO - Magicly yours , - Allen L.
It's a tempering valve, some times the words just won't come ! Thank you - R. Scott