Opinions needed, handrail issues.

About a year ago, my co-worker and I installed an ovalised 1.5"x 2" handrail. On both sides of the stairwell running up 4 flights.

The top flight runs along a pony-wall on both sides. The rail wraps 90* at the top of the flight and returns after 1'. There was no backing installed here. Instead of cutting open a finished wall, I notched the top plate of the pony wall and slid backing down. Unfortunately this did not hold up.

I'm thinking of trying some hilti zip-toggles. Or wanted to see if anyone else has opinions on a suitable anchor.

I have a feeling I'm going to have to cut thru wallpaper that was applied 1 month ago and a painted wall.