It's trivial to set it up for the attacker. If you have a Linux laptop you can set up a redirect for all the traffic on the network through your machine with two commands, then there's plenty of tools that will intercept any incoming HTTPS certificate, replace it with your own, the decrypt the traffic. It sounds like a lot but anyone can set this up in about 15 minutes - that's why I said it's trivial.

The user mistake is just clicking "advanced" then "proceed". I know all my family members would do that without questioning.

Maybe teach them to not do that.