Walmart sells a Coats & Clark thread for outdoors that is labeled as holding up to UV light, although I would think your problem with sandbags would be more along the line of moisture than sunlight. I've used that thread in a Singer 301 for repairing flags that hang outdoors, and also for the heavy webbing belts used for riders in a therapy program using horses. The belts have webbing handles sewn on the sides and back of the belts. All these projects were 4 or 5 years ago under the local 4-H umbrella, and I have not heard any complaints.
Sailrite is a place you might want to ask about suitable thread. Somewhere around here I have instructions for sewing shopping bags from the animal food bags woven of plastic. It was published in a magazine. If I ever find it again, I'll see what thread they used. I think your needle is okay, just overkill on the thread size.