I had your setup and made the following change which has been very satisfactory.
The boiler was using a lot of oil in the summer just to make up for heat that goes up the stack while the boiler stays hot for when hot water was required. It was costing more for the oil than it would have cost for the electricity.
I installed a 40 gallon electric water heater that is fed by the tankless coil to the inlet of the water heater. I turn off the boiler in the summer and let the water heater take over. The electric heater is set at 130 F. When the boiler is operating, it supplies hot makeup water to the heater and the electric elements don't come on.
The system stabilizes the temperature between winter high demand times such as when you are taking a shower.
The system will work just as well with a gas water heater.
I found that the saving of oil, which is now about $2.20/gallon, is about twice what it cost me for electricity to heat the water in the summer. In the winter when the feed water is cold (municipal water is from a lake), the water is heated by less expensive oil.