Another cause of top thread shredding can be caused by a broken needle: a ‘bur’ on the thread path. A ‘bur’ is a tiny bit of displaced metal, made when your needle hits anything but the hole in the throat plate it is supposed to go through. It is when your formerly pointy needle gets flattened at the end. That metal goes somewhere, and it becomes a bur.
Yes, this is just as bad as a bur under your saddle….
You will find the bur on the tip of your needle, and whatever your needle hit. If the bur is on your needle, replace it. If your thread path has a bur on it, you need to buff that out.
Over time, and after a needle break, a bur can form anywhere along the thread path, so you need to search carefully, feeling along the thread path with a nylon stocking over your finger to see if it snags anywhere.
If the stocking snags, you know you have a bur. Buff out the bur with a fine grit (800 or so) sandpaper.