Suggestions for improving these stairs
There are a lot of ways to handle this based on your taste.  The lower 2 treads appear to be cobbled together and if it were me I would tear them out completely taking care to salvage the treads.  Then rebuild the lower structure from scratch utilizing the treads.  Purely my opinion based on a single photo, but I think it would be easier than trying to clean up what is there and it gives you more options to do what you like.  Then, whatever you do for the face of the riser, you can do above the second tread.  Even if it sticks out beyond the face of the wall, it will look like a deliberate part of the stairs which will now be treated and seen as their own unit instead of a mishandled attempt to match the wall.  At any rate, I wish you well with your project!