You have a cycle in your gear mesh that will not be guaranteed to have full contact all around. There is even the possibility that teeth won’t line up and the gearbox will bind, but with your tooth counts, the symmetry of the shaft placements, and enough backlash this is unlikely. However, I’m not sure what sort of effect an incomplete mesh can have on wear. Likely you are just transferring all the torque through one line to the output. You are certainly not going to balance it through both gears driving the output gear unless you have much much much finer control over the backlash.
In that vein, then, the pinion on the middle CIM does not have much engagement with the cluster gears. It seems this is due to space constraints, but whatever you can do to get full face width contact will help.
I really like the footprint. I’d suggest freeing the output shaft from the gearbox, but keeping the bearing that holds the backside of the shaft in the floating plate. This way the shaft can be integral to the drive rails (chain in tube, wheels and all), and the gearbox bolts on, piloting on with the bearing, with no need to ever remove the output shaft. The drive base should roll around real nice with the gearbox removed, instead of awkwardly missing that middle wheel and leaving chains dangling (or alternatively pulling the shaft out of the gearbox and leaving the gear and shaft collar loose). And gearbox removal and replacement is a breeze.
Edit: one last thing, you could swap the floating plate with the middle CIM screws, this way the floating plate is more inline with the forces applied to the output shaft by the driving gears. Gives those standoffs more gear clearance too.