Always better to say what You did experienced yourself... All theories are good and work .........on the papers
What I did is to put a black used water softner tank 13" X 54" holding about 17 gal. I strapped the tank to the basement's ceiling so the warmer place in the basement all year long. A simple valve syst can be add to ocasionaly inject some desinfection product if you are scare about bacteria.
The cold water enters in this tank before the hot water tank as a pre-heating device. The next hydro bill went down. We must remember that, the most electric expensive appliance in a house after the heating ( supposing an electric heating system) is the hot water tank.
And about legionella, the water should boil or be very close to to kill bacteria, so a good periodic hot water cleaning/flushing (a good flushing is more than just opening the hot water tank's drain valve) is the best way to prevent problems. And wash your hands once in a while