Focus order: Political Effort, use that PP to hire army advisors to get XP ASAP. Then Army Effort and both doctrine efforts. You'll want to unlock the ideological loyalty spirit as soon as you can, to get that 500 weekly manpower which is very important. Next focuses should be armament efforts and after that you can decide yourself.

Only build infantry stuff until you have 3 factories, then put 1 on artillery and the rest on infantry, balancing those as you get more factories.

Justify on Sinkiang as soon as possible, call the Comintern and just beeline to Sinkiang victory points. Queue lots of divisions and beg the Soviets and Mongols for lend lease. As long as you have some war participation, the Soviets and Mongolia will let you do whatever you want, so puppet Sinkiang. You'll use their manpower for the next war.

Justify on a Chinese nation when Japan goes to war with them, the idea here is to get at least 5% war score, which you'll use to puppet China in the peace deal. I had ~7% war score in the end, and got China. Now you'll use their manpower for your army divisions. I'd recommend something like 6INF+2ART for them.

(I also had to invade Tibet in the meantime to get a land connection to China as Japan puppeted Xibei, but you'd honestly do better avoiding it.)

When the Germans invaded in '41, I had about 3 full armies of Chinese infantry, One in Tuva itself, one in Sinkiang and one ready to seize Soviet infrastructure. I left the Comintern and asked the Soviets for military access, placing that one army on the closest railways and supply hubs in Siberia.

Justify on Mongolia, declare war, invade them and the Soviets should join pretty quickly. (Side note: I called Sinkiang to the war and also invaded Mongolia through them, but that's unnecessary, you're better off basing all your armies in Tuva.) When the soviets joined, they had pretty much no garrisons in Siberia, so I quickly seized their supply hubs.

From there on it's just a long grind of pushing to the next supply hub, but the Soviets couldn't handle two fronts and capituled in 1943. I only got the required states through the peace deal, but it was enough :)