I agree with you infantry is indeed far too expensive to make and maintain in terms of manpower, although a tank crew consist of 5 men and an infantry squad consist of 10 men, the tank have higher survivability than an infantry and infantry have the highest casualty rate. It's resonable that tanks are cheaper in terms of manpower but infantry should not be expensive, personally I prefer he infantry than the tank using to support them (personal play style)
I play Italy as well, may I suggest to use subs or destroyers and anti-air guns to protect your coast, I know it's expensive but that way an invasion from the sea is impossible, northern Italy is mountainous with fortifications and a defence force they won't easily attack you, it can also serve a backdrop when you fail. This way you can leave your coastline empty, mount an offensive, and sleep peacefully knowing you won't be attack from the sea or land although I can't promise you won't be bomb from planes so make those anti-air.