Good catch, i had forgotten about that cut.3/4 x 3/4 will work. You want one that will cut 3/4in wide you can adjust the dept or the width on the router table. With the 3/4 in bit you can make the dado for the bottom boards or you set the fence to only cut 3/8ths to make the cut for the frame rest. I love my router table it is a lot faster than you might think it would be.
I was looking at a biscuit cutter to join 45s with no exposed end grain. The problem I found with deeps is getting the cup out of deeps when cutting on the miter saw.I have a lock miter router bit that I have used in the past for the end joints. Works great, a bit of a pain to set up to begin with, but makes the end cuts and assembly very easy with very little squaring required. Plus the wood requires just a bit more length as tboth end and side boards are full length. That little bit of difference in length becomes critical on 8' long dimensional lumber.
That said, the last boxes I made (yesterday) I used my radial arm saw to cut rabbets.