I'm pretty sure the optimal way to get the highest number of factories possible is to research-juggle your industrial techs (Dispersed 1 and Construction 1) with an empty slot (basically saving up 30 days of research on an empty slot, then swapping the desired tech in progress over to that empty slot and accelerating the research by 30 days, you can do this whenever you have an empty research slot), and take Rhineland followed by Four Year Plan. If you can manage to start researching Construction 2, Dispersed 2, and 1937 Machine Tools before the Four Year Plan focus finishes, you can use both bonuses the focus grants you on Construction 3 and Construction 4, the latter of which will be finished in 1938, three years ahead of time. This combined with going Free Trade and picking up the Captain of Industry will massively snowball your buildup phase.
The opportunity cost is you're spending a lot of time researching construction tech, so the most likely sacrifice is being behind in infantry tech or navy. This is fine though, since infantry are mainly defensive and a navy is not the most important thing to research as Germany and submarines/destroyers are cheap enough that you can get a fleet going quickly once the research is finished even if you neglected it until later. The only areas of research you need to keep on time or ahead of time in as Germany are Industry, Electronics, Fighters, and Medium Tanks. Land Doctrine is also a priority, and Air Doctrine up to the fighter agility tech in your doctrine of choice should be completed by the start of World War 2.
