Unfortunately maker-spaces (the only kinds of places where you could get cheap access to good 3+ axis CNC mills) seem to have gone out of fashion. CNC mills in general cost about 5 digits just for the machine, and usually that again for the tooling.
I think the closest one can get to learning some DFM without working on a machine would be watching machinists on YouTube. Back in the day NYC CNC was great, but I think they post more hype content these days.
I'm far from an expert on the topic, but I spent a decent amount of time designing for and then fabricating stuff on CNC machines over the last couple years. To my knowledge, operations are cheap while setups are expensive: if you can design a part around a fixed orientation in a machine, it saves a lot of operator time.