Connecting Wood/Electric Furnace to Smart Thermostat
You have two 24VAC transformers here, and that's where the fun starts

Your system is unusual in that it has two 24V control transformers within it -- one for the electric side that powers the relay and contactor, and a second for the wood side that powers the servo motor.  Furthermore, the transformer for the servo motor is half the size of the transformer for the electric side of this furnace (20VA vs 40VA), limiting its ability to power a power-hungry WiFi thermostat as well.  Given that the wood-side transformer has its C wire exposed at the control terminal block and the electric side transformer doesn't, that leaves you with two options:


Replace the 20VA 24VAC transformer with a 40VA version (40VA HVAC control transformers are utterly bog standard) and then tap C at terminal 6 on the furnace's control terminal block, using a 24VAC SPST relay to allow the thermostat to control the electric side.
Tap the electric-side C terminal, likely at terminal 7 on the relay, and then use a 24VAC SPST relay to allow the thermostat to control the wood side. 


Something like this would suffice for this application as far as relays go (technically, it's not UL listed for standalone use, but for mounting into a furnace, that's not a big deal).