The north is probably more valuable than the west. Good luck holding those huge provinces, anyway.
Manchuria's tow-three industrialised provinces hold more IC than the rest of the communist area combined (especially as they seem to have lost Yan'an). Go for those, they are relatively easy to reach on the one-province wide ocean-side road that actually has any infra. If you have any airforce, conduct logistical strikes on the ComChi held, low infra provinces. Their TC must be at least as bad as yours. Use that to your advantage. Whenever you see their troops shuffle around, attack, dislodge, but do not occupy the province (unless its a high-infra ones, then try to send a cav to tear stuff up). They must not be allowed to recover org.
Once you are close to Port Arthur, strike north, eliminate half their IC (former Manchukuo capital and the province next to it), and then slowly grind them down.
Easier said than done, I know.
To be fair, if they were going all out, they'd do something stupid like "All Nationalist provinces in mainland China defect to the Communists"
That being said however, is it possible to build up some soldiers fast enough to counter this?
I think you're having worse luck than anyone else, I usually get a few of those, but not as many as you're getting. But it is realistic, corruption was bad and often troops deserted, basically the Communists were able to easily take over the NRA equipment because so many men ran away
I am not saying these shouldn't happen, but not to the extent it is now, especially when I have managed to keep dissent so low, and I think what lost the war on the front was that the NRA's best units were cut off and destroyed in Manchuria, not what is happening here