I’ve used the self leveler on concrete floors before tile. It is a skill in itself to get it level. When stapling or nailing any material such as flooring. It is common when you hit something hard , the staple can curl and come to the surface of your floor. I would think a leveler that was like acrylic mastic might work better. Hope you don’t have low spots over 1/2 inch. You might try going to a hardwood flooring supplier and ask what the floor guys are using. Most pros do not buy at the box stores. You need a straight edge , longer the better, and go over your floor. Get good with a trowel! You might think about screwing your old subfloor down to eliminate squeaks. Make sure you store the flooring in the house to acclimate. Leave enough space for expansion.
