How would you handle this exterior wall in the bathroom?

Currently framed with 95mm and insulation. Black spots are visible on the stud at the bottom. There used to be a bathtub here, but that won't be the case in the future.

The rest of the house is only framed with 21x95 boards and then plasterboard, so no insulation. Old plastered house.

Skip the insulation for better circulation?
Plastic or no plastic on the inside of the stud?
 
A partially plastered wall with exposed wooden framing and a black cable running vertically, found in a room undergoing renovation.
 
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