You mention 84 pieces for a sign but are you making multiple signs? If lots of signs a special fixture might make sense but for a single sign I wouldn't bother.
I would probably cut the circles from a bar as you suggested. First face the bar with your flycutter, cut the disks but leave 3-4 tabs to keep the slugs in place, chamfer, cut the slugs apart on your bandsaw, clamp each slug in the vise supported on suitable thickness piece of scrap and using a pair of V-blocks. Face the back side to remove all traces of the tabs. If you could get away with slightly thinner than 1/2-inch you could leave tabs 25 thou thick and use 1/2-inch bar stock.
For this sort of low precision job I frequently support the aluminum on strips of MDF to avoid damage to the mill table and secure things with strap clamps.