Based on your description in original post, this is what I'd do. It's about as simple as it gets. This is from Xantrex if I recall correctly but will apply with any brand inverter.
You can take power from an unused 20-Amp breaker or install new one in unused slot, and run it to the new inverter with built-in transfer switch. That way when on generator or shore power, inverter is automatically by-passed.
I'd try to find a place for the AC load panel where wiring from circuits being powered by inverter don't have to be spliced if possible.
Note: If wired like this using a 20-Amp breaker to feed inverter's transfer switch, the total of all outlets downstream connected to inverter will be limited to 20 Amps. Hence, if someone turned on a hairdryer in bathroom and someone else a coffeee maker in kitchen, it will likely trip the breaker feeding the inverter.
Just mentioning this in case you want to be selective on which outlets you connect through inverter.
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