I don't see any particular reason to disconnect the loop for the sweeping.

I rather like to have a summing amplifier stage for the excitation so that we can use it to measure the open loop TF.
The input impedance of the summing stage should be high enough (~10K) such that we don't have the calibration
error between 0ohm and 50ohm signal sources.

AD829 may get unstable when the gain is small. Read the data sheet carefully, and put additional pads
so that we can put additional compensation components as described in the data sheet.