Have a look at some standard assets. You are searching for smoothing. So for example, the script in `Standards Assets/Scripts/Camera Scripts/SmoothLookAt.js` does these things with a lookAt. So maybe you can take a look into that script.
I can't remember exactly, but I was able to get something along those lines in one of my experiments, using the drag rigidbody preset script, with another script that reset its position in LateUpdate so dragging only rotated the object.
This made it so dragging quickly would spin the object and give it momentum to keep spinning. This could be adjusted in the Drag rigidbody script using the spring and damper settings.
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– maccabbe