@tomasenskede
Root cause identified and resolved!
The issue was related to the Digitus Media converter, which didn't support mixed speeds. I had a 10 Gbps connection on one side and a 2.5 Gbps connection on the WAN side of pfSense.
However, after replacing the media converter with a switch that had both 10 Gbps and 2.5 Gbps ports, the collisions and errors ceased.
Case closed, and now my WAN data flows without any errors!