All the RC model helos seem to have moved away from the two-blade teetering in the early days, the current designs are using two-blade without the teetering hinge.
This hingeless(rigid hub) design that models use is virtually the same as your proposed teetering hub with a hub spring, I think.
I have not found any reason why a two-blade rigid hub with flexable blades would not work on a full size(ultralight).
The rigid head has gyro stability in the main rotor.
The rigid head has control moment at the mast.
The rigid head has less parts.
The rigid head has no problems with zero G.
What is wrong with a rigid two-blade rotor?