There are other interesting possibilities, for example, what if all kills made by all Wehr troops were transferred to - get this - KKC. Then, you can purchase global vet using these XPs, not using fuel and manpower.
This is it. Wehrmacht get to keep their global veterancy but have to earn it in combat. As already explained by the Doctor the Wehrmacht faction already has a very high teching cost so removing the need to spend fuel on veterancy will make them a lot more competitive, but not necessarily overpowered. How easy would this be to create as a mod as proof of concept? Then it would be possible to test the idea not only to dial in a reasonable veterancy gain rate, but also to determine if removing the need to spend fuel on vet will result making the Wehrmacht tech too quickly or have enough fuel to spam their vehicles.