Great that you are making changes with your foot position and working to wean off orthotics.
12mm drop to a 4mm difference to do all at once.
Your feet, calf may not be strong enough for a 4mm drop shoe.
There could be stitching or something pressing on a nerve.
Are you addressing the plantar fascitis?
Episode 77: Plantar Fasciitis
Episode 220: Lower Leg Business
Have you seen the rebuilding your feet series?
Pro Episode #3 – The Flat Feet Solution
Rebuilding Your Feet With Brian MacKenzie
Rebuilding The Feet, Part 2
Rebuilding the feet, part 3