Replace them with the same commercial grade all steel ply carcass Bridgestone R238. You get the same size and load range commercial grade tire as the XPS ribs but at a lower price. Their the tops for 6k axles and smaller.
Great selling point for selling your trailer.
The Endurance is just another poly carcass tire and can't be compared to a commercial grade all steel ply carcass tire.
If you go with a poly carcass ST tire the only ST tire that size/load range I would use or recommend is the...Providers..Carlisle HD....Endurance which are at the top in load C/D/E tires. These three ST tires have very few tire complaints.
The Sailun S637 is a Load G load range (14 ply rated) tire at 4080/4400 lbs capacity at 110 psi which requires a 110 psi rated trailer wheel. Way too much Over kill for 5.2k/6k axle .