The problem here is that OKW doesn't have the amazing team weapons that OST has. Axis already has OST acting as the de-facto defensive faction.
If you take away FRPs and don't give mobile reinforcement options to OKW, there will be no incentive for them to ever launch an offensive (otherwise, a bad retreat means that your trucks will be swept away). In that case OST will be the defensive faction, and OKW will become the super-turtle faction.
Why don't you just make it to where OKW doesn't have a frp? 9/10 times I don't build my medic truck outside my base I fear of artillery.
So just reduce battlegrouo to home sector but give meds for free once again.
I mean not every faction needs a frp or a field reinforcing unit right?
This is all in assumption of rocket arty and brace being nerf so OKW has a chance against such units