From what I undestand, between 1933 and 1939, Poland produced around 600 fighters but sold nearly 200 of these to others nations (mainly Romania, Bulgaria, Greece and Turkey). So if Poland decides to not export these fighters and to concentrate on the PZL 24 instead of the PZL 11, Poland can have around 500 relatively modern fighters for September 1939 without losing much money. And to have more AT guns and rifles and heavy mortars, an earlier French loan (let's say in 1936) could finance them. With this money, you could also produce more tanks (250 instead of 150) and buy others (perhaps 100 35T from Czechoslovakia before 1938, thanks to better relationship as said above) and buy planes (100 AVIA fighters could do the trick).
All these improvements, combined with well-trained polish troops (like you justly said), a slightly better battleplan (better located ARMY LODZ, following Weygand's advice to abandon the Poznan salient) could already partially turn the blitzkrieg into a war of attrition, at least for several weeks. The consequences of these could be huge: Poland already buys time but then so is France and Britain. A bled Germany would need more time to recover even if Poland still falls. And considering the Entente was rearming at a frantic pace, it could be enough to defeat Germany in the West OR at least weaken them badly. But honestly I'm just announcing possibilities here and I could wrong.
Me too, I would really like to read a TL on Poland in 1939. There was one about the battle of Bzura. Really interesting stuff but sadly the author stopped after a few updates.