After trying horizontal mortising it quickly became the only way I will make mortises.
This method is fast, accurate and clean.
If you want to use your jacobs chuck, you can drill sequential holes, then clean the mortise with a chisel or even drilling out the uncut portions. This will still need a clean=up with a chisel.
However it works a whole lot better using a router chuck and router bit. Using your Mark V, You will have to make light cuts. One method would be to use the router bit as a drill, then use it to clean up the edges.
I have never understood the reason to make square ends to a mortise. It is a lot easier, and probably more accurate, to round the ends of tenon than squaring ends of a mortise.
In fact my favorite method is to mortise both sides of the joint. Tenons can be make on a long board. They can be cross cut to length later. The long tenons can be rounded with sanding, hand planing, or routing. This type of tenon is called a loose tenon, although they fit very snugly.
Octogenarian's have an earned right to be a curmudgeon.
Chuck in Lancaster, CA