Unsupported outside stair stringer flexes too much
Getting ready to insulate and drywall in our home rebuild. I have a few days left to make things as right as they can be. This stair goes from the second floor to the third floor attic area living space. It is less wide than the stair below by design. Walking my 290 lbs up deflects the stair down to the left unsupported side. I was wondering what can be done (other than losing weight and having smaller people visit the third floor) to stiffen the outside stringer. Obviously the stair tread and riser will help with this. Possibilities include:
1. sister in an additional stringer
2. add steel flitch plates to each side of the outside stringer
3. or both 1. & 2. but to the middle and the outside stringer
4. add bridging every two feet between the stringers
Additionally, there are no joist hangers at the top where the stringers are headed off. Should there be for additional strength?
I have included four photos to show the stair framing.
I appreciate the feedback.