I am trying to make a 1920’s push button type light switch smart with a module wired behind the switch. The switch will not control a load but will be used to trigger devices and automations via the app and physical control of the switch. I want the app to be synchronized with the physical switch state and not simply toggle on/off with every switch state change.
Cheapest method would be with a sonoff device. Basic is cheapest, mini is smallest.
If you want direct link to ST use the basic zigbee device. Less than £10 or $10
You should be able to mount almost any zigbee or zwave relay behind the switch in the box. All the power in and out would go thru that device. The relay almost all have an input you can wire to the switch. When you activate the switch the relay sees the change and switches state. It will also notify ST of the change.
You should note that your switch will no longer behave like an on off switch where up is on and down is off. It will behave more like a 3 way switch where a change of state at the switch results in a change of state of the relay/light.
Something like this would do the trick