Hi Greg. Happy Easter. Rather prefer three strand nylon mooring line over polyester double/triple braid for the same reason when selecting ground tackle, it has flexibility. Please define a welded galvanized sling link; a shackle? If so, do they hot dip galvanize their product? Big distinction between galvanized and hot dipped galvanized in terms of long term corrosion resistance.
Never secure the mooring line working end from one cleat (port), through the mooring ring and securing the bitter end to an opposing cleat (starboard). Rather prefer two mooring lines, one as primary, the other as a backup with some slack in the event the primary fails.
When selecting ground tackle, the maker should spec their product by working load and breaking strength. Our boat displaces 35,000 pounds cruise ready, so I use 5/8" three strand nylon, which is more than adequate. Breaking strength normally doubles the working load.
Regarding the mooring, personal, private or public? How was it secured to the sea bed and what material was used to secure the anchor to the mooring float? Annually inspected/tested? Department of Natural Resource moorings around our precincts rate working load by boat length, with double boat length breaking strength. They inspect them annually, use helical coils to secure them to the sea bed and flat nylon webbing to connect the coil to the float using hot dipped galvanized shackles. This arrangement minimizes sea bed erosion. I've seen mooring conditions where I would not even tie up my dinghy. And others where our buddy boater drifted away due to a tether failure.