How to finish drywall under stair treads and risers seamlessly?
Since your goal is not to use any trim, we can't cover it up. You need something that will look like drywall after you paint it. If you use joint compound / putties it will crack at the seams if the steps budge/give.

What about paintable caulk? It'll maintain its elasticity so it won't crack, it'll seal the edge, and it'll look like drywall after its painted.