Lots of good info here I’m not going to repeat, but I think if you’re going to open home assistant to the public Internet, you should not use duckdns. It’s better to get your own domain and set up a reverse proxy to use https. Or if it’s too technical, subscribe to nabu casa and let them handle it for you.
Flickering like this usually occurs when the controller and lights are not using the same ground.
My first thought would be an air quality sensor but I’m not sure how sensitive they would be to food smells. I’ve never used one but there are plenty of examples online.
A fallback would be a timer, if you know when your neighbor usually cooks, you could turn your fan on during those times with some buffer.
Wireguard is good if you don’t want easy access to notifications or location based automations. Otherwise you need to always make sure the VPN is on.