On the wild offchance that anyone else hits something similar...
It seems my original instance had a legacy upstream WAN gateway still in place.. Of the two WAN GWs in place, neither was flagged as an upstream GW (i.e. a candidate for default in my understanding) in the OPNsense GUI, although checking the routing table in the shell showed it to be correct and expected one. Once I removed the redundant gateway, marked the remaining one as upstream and re-exported my config, all was well after import to the target.
All the best.
It seems my original instance had a legacy upstream WAN gateway still in place.. Of the two WAN GWs in place, neither was flagged as an upstream GW (i.e. a candidate for default in my understanding) in the OPNsense GUI, although checking the routing table in the shell showed it to be correct and expected one. Once I removed the redundant gateway, marked the remaining one as upstream and re-exported my config, all was well after import to the target.
All the best.