Sand or top soil is good. The issue with top soil is clods. When you go to level it with a drag, even a 1/4-inch clod will cut a small furrow into the level surface. Sand does not have that problem.

Which sand? Don't overthink it. If you can get sand with faceted edges, get that. I've never seen that in a bag. Mason's sand might be better than "play sand." Might be. Play sand is certain to be river bottom sand ground over the eons to tiny spheroids which slip around over each other. Once you have grass and roots, it all firms up. I used beach sand once, and it was firm enough to park the car on. However, I let it all die during a drought, and it returned to very soft sand unsuitable for parking the car. It had to be leveled again before watering for grass. Top soil should not have that problem at all.

6 ft x 6 ft x 2 inches = 0.2222 cubic yards.

Thanks, Dennis. I ended up using about 50/50 mix of play sand and topsoil. I have two patches I'm allowing to settle now and will seed soon. I can see it will probably be a multiple year process to get just right, but it's already 90% better than it was.