I like the information you have presented and I agree.
I am looking at this at a slightly different angle. I use a Carrier / Climatemaster geo- thermal heat pump (open loop) and my heat pump has the additional second optional heat exchange coil (desuperheater).
When I first installed it in 2008 I tried using a low boy 50 gallon tank, and it was “tricky” trying to get the original water heater to provide hot water economically year round. With a single electric tank it just doesn’t work well, because you have to lower the temperature of the bottom and top thermostats so you are not feeding electrically heated water to the inlet side of the desuperheater coil in the geo heat pump.
When I purchased my GE geospring (which is currently awaiting repairs for a leaking evap coil), I installed it in the house and began using the original 50 gallon unwired electric water heater to store the heated water produced by the desuperheater in my heat pump.
Heating incoming water in the first tank is efficient and there is no danger of heat being transferred back out of the tank back to the desuperheater. Effectively, when all is working, the water temp in tank #1 is boosted 10 – 30 + degrees above incoming water temperature. That heated water then goes in series to the 2nd heat pump water heater.
So coupled with even a smaller capacity heat pump water heater, incoming water comes in relatively warm and overall water temperature does not cool down too quickly, as I /we shower.
Of course the larger both tanks are, the better everything works. Another important observation: if the first and or second tank is outside in a non conditioned space, you will always be fighting heat loss. The other important things are the use of appropriate check valves in water lines to prevent through the pipe loss when hot water is not being used, (more critical on the inlet side) and if possible getting insulation between the bottom of the water tank and the ground surface.