Best way to save money when hot water is heated by tankless coil oil boiler

I realize that I am throwing money away heating my hot water in the summer with the tankless coil within my 1993 peerless oil burner. Oil is around 4 dollars a gallon and I would guess I am using 200 gallons just for domestic hot water.

Is a hot water storage tank -- a booster such as the therma foil 50 gallon hot water booster, enough or should I ad a fully indirect hot water heater?

The cost of the therma storage tank is $1400 installed and it has a circulating pump.

I also have a gas line in the house. If I can save even 100 gallons a year, this can easily pay for itself in 4 years but am I missing something or is there a better way to go?

thanks