I would consider this scenario:
Build a "tube sign". Which, for us, is essentially a frame built from 1" aluminum tubing & connectors pieces. The frame follows the perimeter of the sign and there would be a couple additional vertical pieces in the middle for added support.
The sign face would be DiBond/ePanel. There would need to be 2 pieces with a butt-fit seam. In this case try to get the seam as smooth as possible, perhaps a little sanding if needed. You could also use a single piece of polycarb, probably 3/16 to reduce wavyness. Fasten to frame with screws/rivets around the outside edge and DF banner tape or VHB on the middle tube sections.
Apply 1-piece digital print to the entire surface.
We then would attach aluminum retainer around the outside edge. This is .040 alum that has been bent by a local shop into a kindof "J" shape (with 90deg. corners).
The retainer hides the screws, and if you pre-fit it and attach on the building, it can hide your mounting hardware as well.