You mentioned unifi, Why not just add some test rules to your firewall and have them block traffic from your HA controller to test things out. This way you can test HA without fully killing your internet. I do this all the time in my multi house setup. I’ve never seen HA stop working for my smart IOT devices when the internet is down. I now use a cell phone to provide backup internet for HA so I haven’t had HA internet issues in a few years. Maybe you might want to consider alternative internet access if you have some critical components outside your home.