not sure which unit exactly you're complaining about, but veterancy bonuses for OKW's tanks are very good across the board, vet 4 & 5 being no exception here. the pz.iv, for example, gets a 20% scatter reduction at 4 stars and 25% better accuracy on the move plus a decent sight increase upon reaching vet 5. those are massive combat buffs against both tanks and infantry that OKW gets on top of the usual reload and mobility boost all factions get with vet on their respective tanks.
On paper ! Panther and Pnzr IV literally the nightmare of RNG Street plus slow like an Excavator. If you cant literally deal any consistent damage Those "nice vet gains are" irrelevant. Its Nature of the Vehicles stats. Thats why It needs to be adjusted.