I have two suggestions to begin with though I am not an expert in terms of FPGA and Silicom.
Looking at the settings of the buffers (128GB) it seems they are rather big for 5gbs traffic inspection.
It looks to me your CPU affinity is not set appropriately.(For example you have only one - CPU 0 dedicated to management, that seems very low.) You can have a look here for some guidance of how to set it up especially in terms of NUMA allocation.
You can also start suricata without rules (-S /dev/null ) and see if the buffers are filled up again.
What are the specific PFRING kernel module settings you use for the NIC ?