The camera manager is literary both, a brand new camera & its manager. Disabling *Auto Manage Active Camera Target *will make the controller ignore any cameras that might be attached to the controlled Pawn and give full control to the Camera Manager (and its own camera).
From now on, you need to provide methods for your own camera movement, this is done by returning values from the overridden *Blueprint Update Camera - *the Return Value tickbox there is key!
In order to pull off a Springarm-like Z-only camera lag, you’d do something like this: (you can use *fInterp To *if you do not like the built in Ease)
Giving you something like this:
I hope I did not mix anything up; as you mentioned, the documentation here is rather scarce so the above are observations & trial and error. Quite a bit of the latter.
Hope it helps.