A 75 gallon water heater will not deliver 75 gallons of hot water. It will deliver about 70% of its capacity within 20 degrees of the temperature setting. This means if your tank is set at 125 degrees, you should get about 52 gallons at a temperature between 105 and 125. It sounds like your water heater is functioning properly - it's just undersized to supply your large tub.

Your tub holds 72 gallons of water. If you wish the tub temperature to be 120 degrees, the number of BTU required is approximately 72,058. To have that number of BTU stored in 50 gallons of water would require that 50 gallons to be at about 173 degrees. Your heater may be limited to about 150 degrees maximum. I doubt that you want to be in the tub water at 120, but the figures above offer a comparison. .
The hotter water would be mixed with colder while filling the tub. The hotter water would supply the heat and the colder water would supply some of the required volume.
With what you have now, you could turn your heater to the maximum setting and wait a couple of hours before filling the tub. It might be satisfactory. And the heater can be reset after the tub session.