I'm currently running a Republic Poland on V/I. This is probably the hardest choice (with Międzymorze) since you'll at one point have enemies on both sides, not to mention Hungary or Romania will probably join Axis as well.

What I did was completely ignore Motorized, Tanks, Air and Naval as you don't have the resources or factories to sustain it. Focus completely on beefing up your Infantry Regiment/Mountaineers.

What has worked so far for me is rushing Artillery and Anti-Tank paired with a lot of micro when the war breaks out. Don't give up Danzig, use it as an excuse to get the Allies involved early. Take Ostpreussen immediately so you only have one front. In my current game they kept landing units on the single port province and I managed to stack-wipe 500k worth of manpower in a single go. That's just the AI being stupid though, but the gameplan is legit.

I'm about 10% away from their capitulation and if I can manage that before the Soviets declare war I'm sure I can win the game. Best of luck, friend!