Is it feasible to detect a golfball at 120 meters using mmwave radar chips like TI makes?

Yes. See our IWR6843 device. You might like to start with a long range people detection example and tune to your use case. You will likely need it to be very sensitive and use the point cloud output.

https://dev.ti.com/tirex/explore/node?a=1AslXXD__1.20.00.11&node=A__AFSylyb3s.Y0XGHAWn4NBg__radar_toolbox__1AslXXD__1.20.00.11&r=1AslXXD__1.00.00.26&r=1AslXXD__1.00.01.07

Best,

Nate