I have indeed used the "hole in cardboard" trick and it works. Only tip is, be sure to wipe the paint off the cardboard frequently, else it builds up and gobs of paint will start flying off the edges of the hole towards the model.
Hand painting is an art that is at least as difficult as airbrushing, if not more so. Not too bad with flats, but still can be a challenge. If you do use hand painting, use very thin coats, maybe multiple coats. Even with flats, a heavy, semi-gloss coat will show brush marks. Hand painting gloss paints is such an art that I know very few modelers who can pull it off.
I notice the price of kits going up much faster than the price of airbrushes, and someday soon an airbrush will cost less than a normal kit!