Yes, adding more sprays depletes the tank faster.
At 68psi you'd still be getting "pretty good" gallons per minute (gpm) through half-inch, but if it's a long run it could become the limiting factor. Add up the rated gpm numbers for the heads (including the main head), and tell us the length of run (including the number of ells) between the water heater and the shower.
Short of installing a newer bigger tank with more storage volume/burner, for about $500-600 in hardware you could install a drainwater heat recovery heat exchanger, feeding both the water heater's & shower's cold inputs, effectively doubling the "apparent volume" of the water heater while cutting the fuel use in half. NRCan lists 3rd-party tested efficiency at a standard 2.5gpm flow rate, but note that at higher flow rates you'll get somewhat lower net recovery efficiency, but even at 6 gpm a unit that tests at 50% @ 2.5gpm would be delivering about 40%, with a very significant boost in apparent capacity. In general, within a product line the longer & fatter the heat exchanger, the higher the efficiency. It needs to be installed vertically, so it's not going to be a solution for first floor showers on slab-on-grade houses.