If you have long PCB traces and want to minimize the resistance, is it feasible/viable to use through hole vias to be able to utilize copper on both sides of the board? If width of the trace is limited, what might other strategies be? Thicker copper? Looking for solutions that are most economic in scaled production.
You could use traces on both sides, or internal traces, in parallel. You could use thicker copper (for example, pcbway has up to 13oz copper available on outside layers and even thicker copper is available from some suppliers). The cost of ultra-thick copper can be quite high, but the process is suitable for volume production. Here's a 20oz board, photo from here:
IPC2152 now contains information on thicknesses up to 30oz copper, as in this board (photo from here):
As well as the cost (setup and unit cost), and limited suppliers with the capability, ultra-thick copper tends to greatly limit the density of connections (minimum trace width and minimum space) even with the anisotropic plating/etching techniques that they use (in an attempt to get vertical walls on the traces).
You can also solder jumper wires onto the traces (or replace the traces with jumper wires) or use bus bars on the PCB. Pulling back the solder mask to allow solder over the traces can reduce the resistance, you'll sometimes see that in high volume power supplies. I don't think the increase in conductivity is enormous (WAG maybe 30% less resistance), but it's almost free.