Hi Sam,

I want to power one over-the-top shower with hot water long enough for my wife and I to take showers one after the other.

That shower has:
4 x 2.5GPM body sprays (Kohler water tiles)
1 x 2.5GPM Shower Head
1 x 2.5GPM Hand shower.

Only one of the two hand shower or shower head needs to be on at any given time. So a total of 12.5GPM.
For this experience, I do not want to worry about not having enough water pressure or having cold bursts of water intermingled with my shower, especially having a cold burst because someone else in the house does something as simple as turning on a faucet.

I would prefer to be able to take a standard shower (non-body spray) and do laundry, run a dishwasher, or have another person taking a shower elsewhere and experience no ill effects.

Tanks are not out of the question, though by my math a 90 Gallon tank at 75% mix will only get a single 12min shower or so.

Our current water heater is in our attic (Texas style). My wife is not a big fan of having mega gallons of water in our attic due to paranoia over a leak or spill (bad experience).

My water temp this time of year is 60-65F. My water pressure at the faucet outside is 55-65PSI. This leads me to the 60F rise (60->120) that I use to look at all of the GPM data on the tankless units.