I will add one thing: a 20m dipole in the attic will do much more poorly than one hung outside away from the house. At QRP power levels you will be able to use very thin wire for a stealth antenna. Something like black coated 24ga. wire will ve very difficult if not impossible to see from a distance. You might consider putting the feedpoint (center of the diploe) at the peak of your roof to keep it pretty invisible and running the dipole out either straight or in a V shape from there.

20m SSB is crowded. In the Extra class subband you might get away with 5W and an indoor dipole. Mostly, though, it will be an exercise in frustration with the occasional contact thrown in. Getting the dipole outside will make a huge difference. CW, as others have already pointed out, is much easier.

Why just 20m? I think you'd have better luck on 17m where there are fewer kilowatts and no contest credit.

72/73,
Caity
K7VO