Hi,

I'm aware the convection is in the same direction that the water is
pumped. The pump will be able to overcome the resistance of the check
valve or bypass valve, whereas the convection currents won't.

Another way to do it would be take the hot output from the top of the
immersion tank to the same level as the bottom of the tank or below,
before going to the other tank.

If you want to fit a zone valve, by all means do so

cheers,
Pete.