Shaun, In your post, it appears that when your stair tread tenon goes through your mortice at your stringer it will be flush with the outside surface. If planed, sanded, and finished, you will end up with a furnature or cabinet grade end product. For a more rustic look, extend the tenons through the outer face of the stringer 1/4" to 5/16" and carefully chamfer the ends of the tenons. Assemble your stairs, drive in the
wedges and chamfur to match. Another option is, extend the tenons aprox. 1-1/2 through the stringer and make small mortices through the tenon, make small square wedges with a slight taper and drive them down through the top of the tenon which will pull the stair tread tight to stringer. Two wedges spaced about 4"to6" apart also looks good. If you wse this method make sure you use a good wood glue where the stair tread fits into the stringer dato, and DON'T over tighten the wedges or you will split out your tenon. Also build the stairs on a flat surface or in the stair opening for a perfect fit, or your stairs will be racked or twisted. Good luck and let us know the end result, John...