Use https://jsonlint.com/ and https://jsonpath.com/ to check your setup.
AFAICT JSONPATH:$.local_temperature should work (without quotes).
Use command topic zigbee2mqtt/TRV1/set/current_heating_setpoint
See Use case: Integrating IKEA Starkvind air purifier into openHAB for a worked example.
Just for reference: TuYa TS0601_thermostat_3 control via MQTT | Zigbee2MQTT