It seems Mesh feature, which is only available with a ZD, may help here. However, to answer your question fully we will need a better understanding of your existing network (topology) and based on that we will see if these changes can be implemented.
I would suggest to open a case with Support, explaining what you are trying to achieve with the Network topology
It is feasible that you could have two Root Bridge 7731s, using two different channels, with right side half of subscribers into one, left side half of subscribers into the other. 7731s support 30 degrees segmentation. You would configure the right side RB distance to the distance of the furthest right side subscriber, and the left side RB distance to the furthest left side subscriber. If using the same pole to mount two RBs, separate them by 6+ feet.