Unfortunately since I don’t have any of the light switches I can’t give much insight on how easily accessible the pins on the ESP would be. The process of flashing them isn’t too bad. The hardest part probably being soldering your wires onto the pads.
My local Menards just started carrying the Wion ones. The light switches are $20 right now. Hopefully it’s not just an introductory price. They aren’t listed on the website yet. I’m debating whether I have anywhere else I really need one or not since they more I add the crazier my wife thinks I am.
Here is a link to my post about flashing/configuring the plug in modules. Maybe that will get you started.