Agree with the advice you have so far. You don't give dimensions but that angled cut is definitely going to be more than a 10" blade can give you. Maybe the short cut that joins it is possible with a 10"? I've built jigs for this kind of cut before. You need a solid one to hold a stair tread firmly and vertical.

I see no problem with finishing the cut with a jigsaw with a fresh blade and then taking a wide chisel to clean it up.

If you watch woodworking videos you'll see that no matter how well equipped someone's shop is, they're always pulling out a chisel to clean up a mortise, shoulder cut, or whatever. Chisel skills put you at the next level woodworking wise.