Or, I can fabricate an interlock like on a house panel.... https://www.interlockkit.com/
InterLock Kits meet all NEC requirements for optional standby systems (Article 702)
I agree with you guys about powering panel via transfer switch, that is the way my house is set up with the generators. I was going to do the interlock device on my house panel when I was going to buy a 240 volt generator, but decided to buy smaller gens and just power 6 circuits.
Anyhow, I need to figure out what each circuit powers. I know GFI will power receptacles in bathroom, kitchen, and outside. AC prep is probably 2nd AC, and recept probably powers bedroom, BH, and refrigerator receptacles.
I'm leaving all options open.