Stripping the StuG if its current duty and bumping the price way up seems liable to lead to Ost having more trouble against medium or light rushes, where the cheaper tag to field a pair really helps. Not that medium spam is common, per se.
Not a fan of letting them upgrade though. If you invest fuel early you should not be able to be gaining vet and having a field presence that you can then just turn directly into a late game tank destroyer while, say, a soviet player has to tech again and wait for their T4 SU-85.
Meds will see re-emergence when heavies will be all tech bound.
You'll have a choice of 1 heavy or 2 meds with no resource advantage to either, so the choice will be to consider contrary to current tech skip that gives call-in heavies clear advantage.