Inverter: Where to start? Subpanel or No Subpanel?

An original schematic of the existing breaker panel would be more useful than a picture of the front of it. Some panels have a natural break in the feeder bus, after the first breaker position, that makes it easy to insert the inverter transfer switch between that point (electrically) and the rest of the panel. You can easily isolate the air conditioner breaker out of the mix and feed the rest of the panel with the inverter. The remaining panel breakers effectively become a sup-panel.