Paula wrote:

There is a small problem with this renovation that we did not
think of: poor ventilation causing severe condensation on the walls
and wrecking the paint.

This condensation problem is especially prevalent in the winter, when
the bathroom is cold and there is more mist generated from a hot
shower.

This sounds like normal condensation with bad paint to me. When the
walls are cold any hot shower is going to cause a lot of condensation on
the walls. What kind of paint did you use?

Something that might help (a little) is to use one of those heater fans.
Run the heater before you shower to warm the room up a bit and then
switch to the ventilation fan.