Easier to do it by hand with a bit of practice. You need a sliding
bevel. sharp knife or scribing point , marking guage, sharp chisel or
two, mallet.
If you try to use a router then you end up fiddling with jigs etc and
needing different ones for different jobs, not to mention the noise,
dust, general inconvenience, why bother it's no quicker, though
perhaps it could be in a production set up for long runs.

cheers

Jacob