Yes, you did post the SCAD, but, I have been using another CAD program so SCAD is yet another "mountain" I have to climb.
Another thing I like about your design is that changing spools is pretty easy. You see other designs where reducer hubs are used on threaded rods with nuts and it would take more time to change out spools. Time may not be a big factor, but, I understand that recent s/w changes now allow for multiple filament colors during a run. I haven't yet had a need for multiple colors. If you were to use multiple colors, it would seem more efficient to simply "bank" the colors before you start. Then, all you would have to do is load and unload filaments when required.
I think I would just make your reducing hubs for each reel and run the reels together on a dowel. Of course, we'd need a new design for the end mounts that attach to the frame.