I'd agree that OKW isn't really a well designed faction. In the early game the main problem is the kubelwagon. When it is buffed and doing its job (being a fast, agile, suppression platform that doesn't suffer attrition) then it is OP. When it is nerfed and it is not doing its job then OKW doesn't have reliable suppression and can't hang with cons/rifles. The nature of the unit prevents there from being a happy medium.
In the later game, their reliance on super armor is equally dumb. In longer game modes where they have easy access to their late game armor they utterly dominate. In shorter modes they struggle to survive the gap caused by their lack of accessible mediums.
I think that relic really painted themselves into a corner with this faction. As for fixes--maybe turning the kubel into a version of the old motorcycle which can cap but not suppress, give them the mg 34 in t1 and replace the KT with PIV ausf J but allow it to be built after any two trucks are set up. (And add KT to elite armor so there's a reason to pick that doc.) I'm not sure that that would work, but it might.
Kubel is extremely important for OKW early game unless you want the only strat to be spam volks. A well micro'd kubel is difficult to deal with, but basically relegates the OKW to one part of the map WITH a squad (usually storm pios) to support it. Its extremely prone to flanking and units in cover. Every push against it results in it either reversing or being destroyed, breaking its suppression. Later game you have AT guns, PTRS, 120mm, halftracks, m20s, rifle smoke, AT nades, and automatic weapons fire to deal with. The kubel gives OKW the advantage in specific engagements in early game that they generally would've lost, but you don't have to fight it. Cap passed it and bait a squad in while also sending in 1 squad for a flank forces it to run or die leaving the infantry it was supporting vulnerable.