Not sure what you mean by “mirroring the temperature setting of the Nest”. Do you want the heating set point temperature on the Nest to be the heating set point temperature for the TRVs so that they shut off the flow to the rads when that temperature is reached? And are you wanting changes to the heating set point on the Nest to be reflected on the TRVs so that as the set point changes on the Nest, your TRV set point follows?
Or are you looking for the TRVs measured temperature to somehow have an influence on when the heatpump system is operating? If that is the situation, your challenge is going to be wrestling control away from the Nest and manually creating Routines to control the heatpump cycles or you may have to raise the Nest heating temperature set point so that the temperature in that location doesn’t cause the heatpump to shut off prior to the rooms with the TRVs hitting their heating set point(s). Which may have the unwanted effect of causing the downstairs area to be too warm.
If the first scenario is what you are looking for, I’ve seen examples of using the Rules API to copy a thermostat’s measured temperature to another device (virutal or otherwise) using the command for setting the destination device’s heating set point. For example, the command for a Nest is “main.thermostatHeatingSetpoint.setHeatingSetPoint”. There are examples in this topic. You could also use the 3rd party rules engine Sharptools which might be a little more user friendly than the Rules API but would run in their cloud and might require their paid tier.