I’ve used a number of different methods to cut mortises, all of which were somewhat tedious. The best router jig I’ve seen and used (except for the $$$ Leigh jigs) is the Mortise Pal. Very easy to use and very accurate to set up. You’ll still have to square the corners of the mortise, or you can round the edges of your tenons.

Rob, Sault Ste. Marie, Ontario