sorry, I didn't read well enough to catch that you already do the calcium and magnesium thing... good luck anyway!

Take care, they are your only pair of feet!
Cramps are caused by the pounding and gripping of the floor and always being on them. All feet need rest. A bath or foot rub can do wonders, especially if you can get someone else to do it for you. During the day you rest your off foot, the one you are not standing on. When competing your feet work extremely hard carrying your whole weight, jumping and twisting and stopping you. I know of no dancer who does not have this minor problem, it is the badge of honor of what we do. The classic poster of the ballet dancers bleeding foot in the broken down toe shoe is exactly the epitome of passion for the dance. Welcome to the club.
When you dance socially and when competing you do not need to pound the floor plus if you do not rush your learning and learn the exact technical way to do it, Repeating Correctly Under Supervision then you will use less strength and more finesse this will then relieve a lot of the cramping as your foot will not need to compensate for improper balancing technique. Your private coach is the only one who will see when you do this correctly for your body. Not every body type uses the same alignment!
This is a long and wonderful journey keep at it and be kind to your toes, they are your 24 hour transportation system.j..

joo

And the morning of a competition to, 4 a.m.

The top of my foot cramped so badly, my toes were in a claw position. Icing helped.

But I've just started to do a ballet class to supplement my dancing, and one of the things she has us do is stretch the feet. Which is not something I would ever thought of before. It's not something I can describe, but if anyone around you has done ballet, they may be able to show you a few stretches.