Stan, we've been struggling with this same issue for years. We often have orders of 40-50 small
signs which we do using the 3mm DiBond panel. A lot of orders for 10"x14", 12"x18" etc.
We have both a nice table saw and also a panel saw. Use the table saw for this chore mostly.
But still, the prep guy has to spend considerable time with a flat file. And that means flipping the small
signs over after making a pass or two on each edge.
I'd like to cut down on that part of the operation.
I once posted a similar question to yours about a stomp shear. I've taken samples of 3mm and 4mm to a sheet metal shop, but their shear didn't perform. (I'd like an edge that needs no further "treatment")