In the main if mould is building up I would be looking at roof tiles, and blocked air bricks, plus any earth over the damp course.
We had some mould and it turned out three tiles were cracked once replaced the only Mold is the town we live in, all mould went.
Since air conditioning units also dehumidify I would not both with a dehumidifier I would go whole hog and get an air conditioner.
Of course you can expect some mould in the bathroom, but mould elsewhere points to something wrong with building fabric. I have looked at heat recovery units, but it depends a lot on house design. The old HRU has two fans and a heat exchanger and are quite bulky. However they will work in nearly any type of house. The new type the heat exchanger is built into the pipe which goes through wall, and look like an extractor fan, however this type needs a sealed room as it only blows air out, and relies on depression in room to draw air back in through the heat exchanger.
Other items what can cause rooms to be damp includes cooking with gas. With a gas cooker you need a cooker hood with a pipe going outside so the water generated by burning gas is blown outside. Also of course gas is less efficient so produces more heat in the kitchen so also need to extract the heat. Using electric to cook with does not produce moisture except from the food itself, so a carbon filter in cooker hob is enough. With induction hobs all heat goes into the pan, so the room is cooler, so less ventilation is needed.
Note not saying gas costs more than electric, because gas is so cheap likely an induction hob costs the same to run as gas. However because a gas hob is really inefficient it heats the kitchen up more, and causes moisture.
My house shows around 60 to 75% humidity always lower in winter, we use electric to cook, my late father-in-law with same house next door but one, shows around 75 to 85% humidity because he cooked on gas and had very good seals on doors, plus never opened a window.
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