Concrete – Drilling depth for 3″ anchor sleeve

Something for your consideration, if you can have the railing modified or corrected. Don't mount it to the stairs at all if you absolutely don't need to, which you don't. If the railing hit's the house & overruns the bottom step or is long enough. Attach it to the building's exterior wall & the ground or pavement at the bottom of the stairs.

Bridging the stairs is one of the proper ways to do a short railing. The other way is for stairs that closely match the width of the door. The railing only stands next to the stairs but is attached to & into the ground's pavement.

With stucco the wall is two thin for tapcon screws, Since you already have a hole I think your best bet is a molly bolt or toggle that expands once inside the wall fill the hole with calking so no moisture or bugs can find thier way in.