Personal thought, NOT as an educated professional, is to use some leather treatment product like Lexol (as the old Greek shoe repair guy in town recommends, Vaseline, on both sides of the strap and then put the straps compressed by the wood slats of trousers hangers for a few days.

The leather should become more supple and "flatten out." To prevent possible goo on the hanger wooden slats or the wool, use waxed paer as a barrier.

You will have to use 3 hangers or 3X the amount of time.

Best of fortune.