> The quote for plastic injection molds was coming out to low 6 figures, and there wasn't a guarantee they would work the first try, so he bought a 3-axis CNC mill and a plastic injection molding machine for the same cost and could iterate faster and cheaper.

For small parts, it’s now possible to buy reasonably priced injection molding machines (under $10K) that take 3D-printed (resin, not FDM) molds.

This is the way to learn and iterate. Mold making is not as easy as it sounds, but it can be learned. Once the mold is working well you can CNC machine a soft or hard tool.

Although I have to admit that even in the large scale manufacturing space we have no desire to do injection molding in house. Working with a trusted Chinese injection molding company is extremely cheap these days.