Guidance for buying large double saddle panniers
Unable to find anything suitable in my price range, and skeptical I would find anything tough enough outside of it, I spent a day sewing, and reinforced what I have.

I used nylon strap, 50mm x ~3m, and double folded nylon thread (so there were 4 threads per stitch) to replace the prior straps. I was able to loop the strap around a seam/strap/flashing which was just through the hole, you can see the threads looping over it. I attached the straps' ends taut to the bottom of each pannier.