Let me suggest an augmented view of this sizing project. I posit the following: No one involved wants a cold shower, or even a cool shower. Maximum shower time is known, as are flow rates of water that might be drawn by faucets and appliances during showers.

With these assumptions, one has to consider the maximum flow during a shower (including other uses), that a BTU is defined as a pound of water heated 1 degree F, the weight of a gallon of water, the tank capacity, and the worst case heat pump heating capability. Water flowing in is at some ground temperature depending on location, and water flowing out should not be less than 120F.

So at one limit, hot water flowing out can be replaced with cold water which is reheated with sufficient power that there is no temperature loss. (This can be simply calculated.) Designing for this capability will be believed by most to be overkill, so how close one wants to come to it depends on level of physical or verbal discomfort one must endure if the shower goes cold.

At the other limit, hot water is flowing out and replaced by cold water with no heating, and, ignoring stratification, the temperature is falling from hot to cold exponentially. with a time constant of the rate of heat removal divided by the thermal capacity of the water at the hot to cold difference in temperatures. If the water tank is at 140F. hot water is regulated to 120F, shower is taken at 100F, and cold water is at 50F, a regulating shower control will start cutting cold water when about 20% of the tank has been used, and won't even supply 100F water after about half the tank is used.

Given the variation in heat pump behavior over the environmental conditions, it might be most useful to obtain from candidate manufacturers the predicted tank temperatures for various flow rates and times. For purely electric the behavior can be calculated, or simulated with Excel or Calc.

A 10 minute shower at 2 gal/min uses 20 gallons of falling temperature water. Can you keep that above 120F, or at least above 100F? Can you keep it hot enough for 2 to 4 serial 10 minute showers? Do you have any shower "baskers" in the family, or are Navy showers mandated? There are too many personal aspects of family hot water use to just accept generic opinions.

I assume you've read the installation instructions and noted that hybrid water heaters (a) can't be installed in a small water heater closet (like a conventional tank heater), (b) require a condensate drain, (c) make noise, and (d) cool the surrounding air.