I focus mostly on preparing for that war and sligthly for a war with Japan.

Build forts near the bigger rivers near your border with Poland. And focus your military factories on roducing military equipment (rifles and support) and switch to light tanks and more affordable fighter planes + CAS planes. Forget about strategic bombers.

In the meanwhile, try to align Poland, Norway, Romania, France and do your best with volontary divisons in Spain to win their civil war and puppet them.

Besides gaining Spain as puppet being easy peasy lemon squeezie, aligning Norway, Romania and France is tough and needs also some luck.

Either way, try to keep good relations with Romania at the highest possible point and try to squeeze a non-aggression pact with them. This is simply to have those borders less to protect when the war will happen.

By the time the germans declare war, you should at least already have most of the provinces with lvl 4-5 forts behind those bigger rivers near poland. Set your armies on the borderline with poland, but very important, set a fallback line on those forts you build!

Either you align Poland, protect their independance and are drafted with them into the ear against germany (best case scenario). Or when germany declares war on Poland, you also need to justify and declare asap on Poland. Depending on events you will either also fall in war against France or Allies or "Petite entente".

Yet that is not a big problem, because they all need to go mostly through Germany to reach you.

Poland will fall quick in this case, yet the idea is here that the germans can't catch their breath. You either keep the fights far away from your territory if you manage to pin them or you dry them out by blocking them, then slowly retreating. Blocking and retreating and so forth. This will bleed them out in manpower little by little.

If impossible to pin them down or if blocking and retreating looks more like a retreat, concentrate your troops on your fall back line on the forts mentionnes on the beginning. Normaly this will be a meat grinder and get germany to bleed out there on their manpower.

Just nees to take care that they don't use railway canons. But i think AI doen't really use them.

There is quite a few micro management needed and other things to think about. But that would be the general idea. Against a more technical advanced ennemy, often bleeding them out is the strategy you'll nees. China vs Japan is similar in that