I really need to route it because I need to plunge into it and don't
want the slot the total length of the board.

Been a long time since I've done this, so can't remember if I can
route that wide a slot (3/4") -- just 1/4" deep or even less, just
enough to fit another piece of 3/4" plywood into.

Do they even make a 3/4" bit or would I have to do multiple passes for
both the width and depth? (yuk).

thanks