Posting the solution here for anyone that runs into similar issues...
The problem turned out to be hardware-related. The Chelsio SFP+ NIC was identified to be the reason behind the TX drops, and after working with Netgate TAC confirmed the best course of action was to use a different NIC. There are an number of alternatives available out there, however we chose the Intel X710 offering from Netgate to ensure compatibility with the XG1541.
Once the cards were swapped out, we ran the same test and confirmed the TX errors were no longer present. As of this writing we also successfully deployed TNSR at our event for the first time using the new NICs, and it performed beautifully.
One thing important to note: After getting everything configured we needed to support our event (VRRP, NAT, bonded interfaces, routing, etc.) things generally behaved as expected (i.e. connectivity existed). When observing the arp table on switches that each router connected to however, every couple of seconds the VRRP MAC address flapped between the primary and secondary routers. Per Netgate documentation this was apparently related to the Intel X710 NICs, which we resolved by adding "devargs disable_source_pruning=1" to the configuration that brings physical interfaces into the dataplane from the host OS. (Reference https://docs.netgate.com/tnsr/en/latest/advanced/dataplane-dpdk.html for documentation)
