TL;DR: Would it be code compliant and safe to build a narrow wooden box to run along the base of a garage wall to protect runs of numerous electrical wires?
I'm going to be adding a load center subpanel inside my garage (fed from a 50 amp breaker in the main panel with a 4x 6-gauge cable) to make it convenient and modular to add electrical outlets for shop tools. Initially I'd like to design the new subsystem to accommodate six independent 20-amp GFCI outlet circuits and overhead lighting, but I don't picture adding them all at once, so I want to design it so each addition will be easy and have minimum cost as I can get to it. Although it's a lower priority, it would also be nice to have it easy to swap the 6/4 feeder cable for a larger gauge if I ever want to feed the subpanel from a larger amperage breaker.
For context, the main panel is combined with the service meter, on the exterior of the attached garage. Where I want to locate the subpanel is on a new piece of plywood over the surface of the drywall, on the opposite side of that wall, inside the garage. It will probably be a few stud bays over from where the main panel is affixed.
Obviously, I want my solution to be safe and code compliant. My location is Lehi, UT. I do have a specific right/wrong question related to a specific solution idea (though, of course, I also welcome alternative ideas in the comments).
I prefer surface mount for both the boxes and the subpanel to minimize the need to remove and rehang the drywall or to fish wires through studs.
I have already begun a design using a single PVC conduit for running wires (It's inexpensive and easier to modify, and I'm less concerned about toxicity of burning PVC in a garage than I would be in the house). I like the idea of a single conduit running along the wall base to carry the wires for all future outlets. I have found charts (such as [this one]) that give appropriate diameters for the number of wires, but at those sizes it seems more difficult to find the right conduit bodies at a low cost. This difficulty is aggravated if I factor in the large feeder cable for the subpanel--I don't mind a separate conduit for it, but because of details of the situation that I won't get into here, a single solution would be nicest.
So an idea I've just had is to build a long narrow wooden box along the base of the wall where I'd like to run the wires. I've picked up a notion that wires and cables outside of conduit can be permissible in spaces such as a garage, but that code does require that the wires not be located where they could easily be damaged. So this box would be a solution for running the wires as if outside of a conduit, but also protected. I would use conduit to protect short runs outside the box (such as for the outlets and for the feeder wire to the subpanel). I could either build the box on the surface of the drywall, or cut out the drywall and give the box access to the (insulated) stud bays (I just worry if that would bring code sections to do with framing and fire brakes into play). I would make the cross dimensions of the box large enough to accommodate all the outlet wires, the ground wire, and the 6/4 feeder wire all at once. I could unscrew the front of the box for working with the wires, but otherwise it would remain secured. I would secure the wires inside at regular intervals, essentially as though they were inside walls, just without studs and corners in the way.
So, my question is: Can anyone foresee points of applicable electrical codes that this would violate? Aside from the letter of the code, can you foresee safety issues with this solution?