Code compliance of custom wooden surface "conduit" in garage
This sounds like a good use for a heavy-duty surface raceway
The situation you're describing (several circuits following the same path, with future expansion as a consideration, while wanting to keep things neat and tidy vs. the &quot;unfinished&quot; or &quot;industrial&quot; look of a typical conduit installation) sounds like it'd be a good use-case for what's known as surface raceway (called &quot;Wiremold&quot; by electricians after Legrand's trade name for the stuff).  However, as you have noticed, the Wiremold found in home improvement stores (typically the 500 series, or equivalent nonmetallic product) is designed for low-profile work in finished spaces, and thus only has the capacity for a couple of circuits maximum.
In particular, since this is a garage, you'll probably want to use a metal surface raceway system for durability's sake.  Legrand has a suitable product in their 2400 series, although you'll need to mount your receptacles in device boxes instead of inline in order to get all your wiring to fit.  If you want the &quot;in-line&quot; look for your receptacles, you'll need to upgauge to the 3000 series Wiremold, which provides enough space for well, just about anything you wish to do inside it.
Note that you'll want to review NEC 386.22 for fill and derate requirements, as they are different for the larger-capacity surface raceways than they are for ordinary conduits.  In particular, raceways with a cross-sectional area upwards of 4 in2 are allowed to contain up to 30 current-carrying conductors without invoking thermal (ampacity) derates, unless you exceed 20% of the raceway's area with your wirefill.  Also keep in mind that inside surface raceways, you will want to run individual stranded THHN wires in order to keep fill and working manageable, instead of using big, stiff NM cables within the surface raceway.