Hmm...
You haven't mentioned whether or not you use orthotics, or modified foot wear. If you do, does it help?
Does the Physio you see do manual work with the foot/ankle region? As you perhaps already know hands on manipulation is primarily needed here. Naturally in Pes Cavus the foot remains arched meaning that on the base or plantar side of your foot, the soft tissue will naturally shorten, tighten, and adhesions may occur. This would obviously require regular care involving massage, perhaps heat therapy.
You mention several previous ankle sprains, I wonder if any ongoing treatment is being done for this relatively hyper mobile joint.
Hopefully someone on the board who is a foot specialist can contribute.