Would my saying, "If they were to make the 'awrah of a permanently unmarriageable woman (mahram) perpetually like that of a non-mahram in front of her mahram," be considered disbelief (kufr) if the intention behind it is a wish due to being fed up with lowering the gaze?

Scholars have differed concerning the awrah of a woman in front of her mahrams. The preponderant opinion for us is that her entire body is awrah, with the exception of her extremities such as the head, forearms, and feet. Thus, what was mentioned in the question falls under awrah according to one group of scholars, and according to our preponderant opinion. Those who permitted the uncovering of what was mentioned in front of mahrams only permitted it when there is no fear of fitnah.

Wishing to adopt the opinion that states those limbs should not be revealed in front of mahrams is permissible, and it is the preponderant opinion for us. Merely wishing to permit the forbidden or forbid the permissible does not constitute disbelief, even if it indicates a deficiency in faith. Therefore, there is no disbelief in what was said.