Is the panel in the garage full? Your new requirements are not that big. Why another panel in the garage?
90A is a stupefying amount of A/C, even for a home that large, even for Phoenix. But that would be just about right for a heat pump with emergency heat for Nashville. Typically that would be 30-40A of actual heat pump and 90A for resistive electric emergency heat (the emerg heat only runs when it's too cold for the heat pump to function, so they get to overlap).It would be pretty easy for me to take that 30A circuit and just make that the source of a new sub-panel.
One of my original questions was concerning the 90A breaker that feeds the house panel. it does everything but the AC units which are run off their own 90A breaker. Is 90A enough for a 3,400 sq ft home? I guess the only big items are dryer and oven.