I decided I'm not going to do a backplane or anything like that because its just extra money. I'm just going to use DIN rail PCB mounts for each of the cards, because I think that will be the simplest solution. Luckily I only had one card laid out, Dimming, which required a redesign. When I started relaying out the board I thought I could just move a couple things around but I ended up deleting everything and starting over. Most of the reason for this was I finally updated KiCAD and its has some fairly substantial changes since I last updated it, mostly to the way it handle footprints. This is what the board should look like now: