Keeping outdoor deck stair stringers from splitting
I've never had that problem either.  But you could use long screws, say 6". The stringer would be over bored the first 2-3", then piloted for the remaining depth so that only the distal 2" of screw thread into the stringer. The aim is to get the thread to bite into only the solid length of the stringer, not the triangle.  Fill the bore with caulk to keep water out, then drive the screw. 

I prefer my screw heads flush or 1/32" proud, never counter sunk.

Alternatively, you could use lengths of angle iron.  The stringer screws would go into the face of the stringer, not the edge. The tread screws would go into the bottom face of the tread (and be hidden). See: https://duckduckgo.com/?q=angle+iron+to+attache+treads+to+stringers&amp;iax=images&amp;ia=images