With native ST Routines, there is no mechanism to compare outside temp and thermostat temp to check if they differ by some amount. Also, you can only check for explicit values being equal to, above, below, etc. I’m not familiar enough with the Rules API to know if you can accomplish what you want.
I do know that the 3rd party application Sharptools has a very robust rules engine that can likely do what you want. It might require their paid service which is around $30/year and their rules are executed from their cloud.
Unfortunately, Google blocks SmartThings from exposing Nest thermostats from outside the SmartThings mobile app. As such, they’re not accessible in SharpTools or other SmartThings integrations.
That being said, you might be able to use a SmartThings Routine to map the real temperature and thermostat setpoint from the Nest device over to a Virtual Thermostat device. Then you could create your rule using the Virtual Thermostat device.
There’s a post in the SharpTools community listing some of the popular virtual device drivers: