I've successfully used the Bluetooth Info action within tasker to determine when the watch is disconnected via BT (Autowear not needed) regardless of whether it's still connected via WiFi and fire an alert on the phone when the watch is not connected via BT. An app called Cerberus can be used to get alerts on the watch when the phone is disconnected.
Forgot My Phone App
Make both your phone and watch alert you when they're disconnected from each other