I have used flex band for several years in my 727 with little to zero problems.
I used raptor trans carbon fiber flex band for about 100runs and replaced because it started to bend where the strut goes. Surface material was still in good shape.
I replaced it with raybestos pro series band and it lasted about 10runs. Then it snapped in two pieces. It broke from the first row of rivets.

4.2 lever, 11% bigger sonnax servo, 165psi griner vb.
Maybe try 3.8? Still should work under 900hp?
I ordered new red one from cope and it looks beefier. I know i should have used rigid one but no money at the time (never got too much )

How long is your band lasting?