I have always done miter corners with a miter sled on the table saw.  Then cut a slot with the table saw to add a spline.   But this time I had some very large window framing to do and the pieces were too awkward to handle on the table saw.  So I came up with this method using only a router.  A fixture, clamped to the board, is used to guide a router to cut the miter.  The same fixture is then used to cut the slot with a router.   Demonstrated in this video https://youtu.be/HQWZqMmoN3g