> Most people perceive complexity as things they don't understand.

I don't think this is true. For example, as a math teacher, I couldn't do a very good job predicting how easy or difficult students would find particular problems. But I could easily predict which problems would be easier and which would be more difficult. I could do that even though I personally understood all the problems.

I don't think difficulty is complexity. For example, the bitcoin mining protocol complexity is the same but the difficulty goes up or down.

I'll attribute difficulty to the energy required to resolve a system. For example, pulling weight. The complexity of the action is the same. But the difficulty depends on the weights to pull.

Complexity is difficulty of understanding. In the context of mathematical problems, that is the relevant kind of difficulty.

It seems you’re vastly misusing the words and their contexts here.