I might be wrong, but there are no lightweight beam forming Radar commercially available at the moment.
There is some experiments with this one , but that is still a WIP:
Finally Texas Instruments mm wave radar (3D Radar) IWR6843AOPEVM is working as proximity sensor
Other issue deals with multiple proximity sensors, there is no way (for the moment) to merge all sensor at the Flight Controller level. You will need to work with a Companion Computer level to control , merge and genereate the required signals to the FC.