I usually do the basic Airbnb for a month and then ask the owner off Airbnb, as well as the building manager / people in the building if there are other apartments at a cheaper price
I’ve never tried Facebook groups or local sites but the prices don’t seem too different to what you get with this method, at least in Bangkok and Philippines
I’ve never done unfurnished but I assume people buy super cheap ikea furniture, shipping is probably super expensive