What is the temperature setting of the 50 gallon unit? If 120 degrees and the tub holds 80 gallons or more, that explains the problem. Water cools even while a large tub is filling. Adjusting the heater to the highest setting results in the water having much more stored heat, permitting the addition of some cold to fill the tub quickly to satisfactory level and temperature. The on-demand heaters frequently have flow rate and temperature limitations that result in very unsatisfactory operation for filling large tubs. I know of one case where a separate storage heater for the tub was necessary. Since it was not used every day, they turned on the heater about 2 hours in advance only when the tub was expected to be used.

To put it another way, first determine the gallonage capacity of the tub. Then decide on a temperature for the water in the tub. Add at least 10 degrees to that temperature. Using the final temperature figure and the gallons capacity, calculate the BTU content of that much water at that temperature. In the case above, the extra heater added had to be in a crawl space with limited height. We found a 30 gallon model that would fit and it also had the required BTU stored if set at 140 deg.

The space constraints are the least of your worries.

Do you already have a huge 220V wire under there on a dedicated circuit AND the capacity in your panel/ service to handle the huge load an on demand will place on it?

Would probably be easier / cheaper to get a second heater + timer to turn it on a bath time and fill the or really crank up the temp on your current heater and get a tempering valve for the whole house.