Here is one of my 9.5 mm UNBREAKABLE leashes. Only the 2nd failure in 4 years. However this one might have been more than 2 1/2 years old definitely a bit rusty. Should have swapped out sooner. Good news was it broke in the middle not around my homemade terminal tackle.
I am leaning now towards double leashes in bigger conditions. Go with 7 mm but double down on the leash. It takes a bit of artistry to get them to work doubled up e.g. make them equal lengths, reconnect at the swivel, disregard the swivel and attach one to each of two leash plugs . . .
I find that the failure in the middle tends to be pretty random. If you double down you diversify the random factor. Still probably best to swap them out regularly.
I am still loving the thimbles (white hoops). All the terminal tackle is reusable so when a leash does break or needs to be replaced I just rebuild. No need to spend $40 every time.