Your cheapest option is low cost mechanical ventilation.
Get a sparky to install 250mm ceiling exhaust fans to your bathrooms and WC's.
Cost with easy access should be around $200 and cheaper if there is more then one at same time.
Have them wired to the same switch as the lights so...
Call a builder and ask for a hypothetical scenario.
I quizzed a builder I do work for about this about 18 months ago.
Similar situation your looking at if I was to assume your looking at knocking out that rear brick wall and building additional bedroom.
He said approx cost would be...
....Must, drag, myself, away from reading the comment section of that article.
It's like being sucked into the YouTube comment section vortex. The first 1 or 2 can be entertaining but from then on the compounding negativity makes your head hurt.