I found this excellent lecture from IIT Madras yesterday, watched it end-to-end, and filled in knowledge gaps that i had from reading about AGC circuit designs.
Using the design example at the end of this lecture, with a number of modifications so that i could implement it with an opposite polarity variable gain IF amp and use the single-supply LM124 op-amp, i have completed the circuit shown in the photo above.
As explained in the lecture, it is a PID controller. It starts to take control of the gain just before saturation of the entire RF signal chain. The RF gain control on the front of my radio will allow this gain control point to be dialed back by the user if desired.
I am very happy with this audio derived AGC. It's performance is excellent! I will post schematics of this and the rest of the radio eventually once it is complete.