A method and system for dynamically controlling connectivity of a user equipment device (UE) when the UE has standalone connectivity with a first access node under a first radio access technology (RAT). An example method includes the UE detecting that the UE is within threshold strong coverage of a second access node under a second RAT, and the UE making a selection between (i) responsively handing over from the first access node to the second access node and (ii) having dual-connectivity with the first access node and the second access node. Further, the example method includes the UE informing the first access node of the UE's selection, perhaps together with an inter-RAT measurement report reporting that the UE is within threshold strong coverage of the second access node, and the first access node responsively taking action in accordance with the UE's selection.