A) Reduce the number of miners per controller. For best performance, no more than 2 miners per AUC chain. It will drastically lower your number of Stales.
A couple reasons for that: the main one is that the RasPi LAN circuits goes through the USB coms. BTW, RasPi-4 fixed that and uses a dedicated connection between the network interface and the CPU - part of the reason the have better performance. The other reason is that is takes a fair bit of time to send/receive data from a miner and having that many slows things down a lot. That is the root cause of Stales.

B) Use a program such as Awesomeminer to monitor the miners. Along with looking at each cluster on a controller as 1 big miner, AM has a toggle to let you look at each machine by itself. I highly recommend it. Their support thread in the Forum is https://bitcointalk.org/index.php?topic=676942.0