"Serious question: have you tried opening the windows? When it's cold outside the humidity is low, so if there's no rain actively coming in the windows opening them reduces your humidity."
Yes, I am a big fan of opening the windows but we aren't home between 730am and 6pm during the week, and as it has been raining a lot you cannot always have windows open let alone leaving a window open which is visible from the street. Depending on the weather, unfortunately over the last two weeks I'd say the windows have only been opened a few times on the weekend.
Depending on your situation, the outside air on a rainy day could be drier...
Take for example your humid house, 21c at 80% RH. That equals 15 grams/m3 of water.
Outside raining might be 12c at 80% RH = 9 grams/m3 of water.
If you let that colder air in, let it warm up it contains the same absolute moisture but at 21c it is now 50% RH.
Have a play with this calculator yourself https://planetcalc.com/2167/ Make sure you set the pressure to something realistic such as 1010 hPa.