The interlock kit looks pretty straightforward. It’s essentially doing what I was describing in an idiot-proof setup, right?
I don’t see how pushing 30a into a 50a breaker could cause an electrical fire. My generator is only capable of putting out around 3000 watts max. How’s that going to overload a 50a breaker or the wires leading to it? I get that it’s easy to swap the breaker and will probably do so to simplify things, but I’m curious about where the risk is here.

Note: I get the linesman worry, but that’s not really a possibility given my use scenario. The power is being shut off preemptively by Edison during high fire danger events. No one is working on the lines at these times.