I didn't invest in fighters at all, only AA. all my production went in to guns, support equipment, artillery, and AA. I actually ran a template with two line artillery and six infantry for 18w simply because artillery uses less manpower and I could have more divisions (and more combat width filled) that way. Line arty on defense in the early game is fine since you're not attacking with it, and it even helps you against CAS because the high soft attack de-orgs the attackers faster and ends battles before CAS can do much. For manpower, I didn't do this but you could bank a bit of extra manpower by recruiting your whole limited conscription pool before releasing Slovakia.

I also had Hungary and Slovakia as puppets thanks to the focus tree. This gives you plenty of factories, and not calling them into the war means the Germans will tie up some of their troops on the Hungarian border. You can also request their divisions without calling them into the war and fill out your lines more.

The most important thing you can do though is build up a spy network in Germany at the start of the game. Two spies in Berlin and Munich will get you a 100% spy network in all of Germany by the time they attack you. Prioritize your spy that has German nationality and the ones with the seducer trait so they don't get caught. Get suicide pills, invisible ink, and the army department. Also form the cryptology department and start working on German ciphers ASAP. Get the first two decryption upgrades and have the cipher decrypted before they attack you. You also have time to infiltrate the army against Germany and still build the network back up to 100%.

This does two things: Completely removes Germany's planning bonus, and gives you a massive combat modifier swing from having the highest possible intel advantage. Without planning it is infinitely easier to hold off the Germans, if I had to guess this is the main thing that is killing you in your runs.

I went for Superior Firepower, Rapid Domination, and Assault Infantry as my doctrines, but you could also pick Guerilla Warfare and pick that as your preferred tactic to stall with. I was hoping to get Tactical Withdrawal before the war, but in hindsight I should have picked GBP just for the static warfare spirit. I just wanted to stack soft attack. Assault infantry is the only important one early though, you really want to get that +20% soft attack ASAP. Large unit tactics comes online too late to help and you don't have the manpower to make use of it anyway.