What's the best strategy for mouse drag to rotate camera with cinemachine Free look camera?

I’m not an Input system wizard, so I can’t really help you there. If you can’t convince the Input system to do it, then you can intercept CM’s calls to it and do it yourself. Have a look here for an example: How do I make a CinemachineFreeLook orbiting camera that only orbits when the mouse key is down?