Your plumber sees a big payday ahead.
Do a cost tradeoff to include consideration of putting an 80 gallon or larger tank type heater, maybe even an electric which has low temperature loss, in series with your high recovery gas heater. Operate both heaters at about 160 F and put a tempering valve after the second heater set at about 130F. In fact, for the space and cost of the indirect setup, you could put in two 80 gallon electrics.
You will do very little heating with the electric heaters. You could even set the electric temp about 10 degrees below the gas setting and they would only use energy when after you have a very high demand. The electric heaters are largely storage tanks with temperature control.
Variations on the theme would be to add another high recovery gas heater in series instead of the electric, or to put in a small circulator to let the gas heater add more of the total energy for recovery by circulating from the electric to the the gas heater.
If you are in an area where you need space heating, you could get a unit that would combine water and space heating with an indirect heater.