I already spelled out why OKW floats, it's a combination of the fact that you can't spend MP on support weapons (because there are 3 max you can build if you have the right doctrine) and the fact you can't spend any MP on tanks either because you don't have the fuel.
There isn't some weird secret to why OKW floats MP, it's clear as day. And adjusting the price on Volks would need to come with an increase in performance (which I assume you don't want).
Caches aren't a solution to the problem, merely, a way to helping alleviate it. But as I have said multiple times Opal trucks are a much better idea.
The funniest things is that it was the first 1's game Iv ever played lol.
It doesn't matter OKW floats manpower. Giving a way to reducing manpower income by neglecting resource penalty that was built in into the faction design it's not a way to fixing this.
As we all know OKW floats manpower, this is true, but conclusion we get from this fact is that OKW unit are simply performing very well.
I would say the reason for MP float is that reinforcement cost for basic units is too cheap to what they are able to achieve. Volks in particular, which doesn't make sens especially that Volks are almost equal Conscripts early game and exceeding them late.
To conclude as it has been pointed out by senior strategist here before, the way to fix OKW MP flood is to make reinforcement cost of some units more expensive not to allow them to transfer this MP flood into additional resources and making OKW even stronger.
Our game actually have highlighted how ridiculous this float is. I could have played better, I admit but that doesn't change a fact that every other faction struggles with replacing losses while OKW enjoys stable stream of Manpower.