I’ve been busy building some inexpensive WiFi smart switches using the small cheap Nodemcu boards, but just work so well!!. I use it to trigger themes and was < £10 in parts. It uses the free IFTTT (If this then that) service which works with Smartthings and many other other things to trigger events. They are small and can support up to 6 switches and just needs a micro USB cable.
Its been working flawlessly for a month now and handle Wi-Fi outages nicely by reconnecting.
Once the simple switch is built and flashed with my code it sets up a WiFi SSID hotspot which you can connect to via a browser to set up the switch. Theme names can be set as can the IFTTT and Wi-Fi credentials and even change the SSID name and password for multiple switches in the house. Each can be upgraded later over WiFi but clicking firmware upgrade with no need to connect to a computer.
I’ll post some more details later and more full instructions on GitHub!