As Frank said, don't mix domestic and hydronic water systems.
That said, I would get rid of the tank heater anyway. I helped a friend with his system and we used an electric 11kW on demand style heater to supplement the solar as needed. Works great. We did end up modulating the temp of the heater as it was getting too hot. With a smaller heater (the same company makes a 7kW model) I don't think we would have to do this. We also had a make-up water connection to the main pressure water system, through a check valve, to prevent a leak from ever draining the system.
Been a number of years and the system has done very well. The system is very similar to what you have sketched.
Jonathan