Why does my oil-furnace's on-demand water heater go tepid and then return to full heat?
Years ago I had one of the first high efficiency gas boilers. It supplied an air-handler for heat and a heat exchanger for hot water. Because back then this kind of boiler would breakdown about 2 or 3 times a year, I installed backup electric heat in the air-handler and a backup electric 30 gallon hot water tank.

I plumbed the hot water tanks in parallel with separate valves to both and normally just kept all valves open and the breaker off to the electric tank until needed. It was amazing how the two tanks acted as one with just gravity flow for one tank to heat the other. If I hadn't experienced it for many years I wouldn't believe it could work so well.

If you added a small electric tank, you'd increase the amount of stored hot water to buffer the times of high usage. And you wouldn't even have to run electric to it, but if you did you'd also have electric auxiliary hot water.