I feel like by that time it is too late. It seems like a pretty dangerous skill to learn and learning under time constraints would be rough. But I can see the downsides.

Honestly, that's my feeling too. The good news is that not every single person applying the munitions would need to know how to manufacture them, simply to deploy and arm them. The Russian's use of anti-tank mines against Thrid Reich railway lines comes to mind - the conscripts carrying the mines, locating the rail lines, and planting them to destroy trains and railway infrastructure weren't homebrewing them in a shed at home. They were supplied by the state and trained on how to use them, in a way that maybe similar to how modern army forces know how to use Stingers/MANPADS but not how to build them or the components.

Cool