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?

Broke 1st one after about 5 years daily use plus racing . Got maybe 18 months 2nd one . Both were flex with 5.0 lever . In december i went solid kevlar lined band and 4.2 lever . Smoother crisp shift , hoping it lasts well this time .
Griner rmvb

Tex

That was a bad band choice if you used Kevlar on the front band , they don't hold up well stopping a spinning drum. they are fine for the rear band because they don't apply on a spinning drum but not on the front .