I think your best option is caulk and paint. For runs wider than 1/4” use backer rod before caulking to ensure it doesn’t fail over time. If you run painters tape on the stairs before you can ensure a perfectly clean line. Lots of amazing videos on YouTube about how to accomplish this. The only other option I can thinking of is to run small quarter round edge molding— but that would require saw, nail gun… and then still caulking and painting for a perfect finish.
Used to work in commercial construction.. the saying was “If you can’t fit a cat in it, caulk it. If you can fit a cat in it, put the cat in it and caulk it.”