I think the best option here (except WC51 and M3 for obvoius reason) is to bring all doctrinal vehicles to the system "call-in with tech requirements", which was applied in the september patch for part of doctrinal vehicles.
True about call-in they are currently a mess. Some require building other do not, some require CP other do not and some require both CP and buildings (I am not talking about Super heavies that are separate issue).
Another difference here not mention is that some units can not be call-in if a structure is destroyed and that makes little senses. For instance Ostheer Tiger is always available while Tiger ace/OKW Tiger require a building to exist on map.
Imo most light vehicles/transport vehicles are not so much of an issue but an attempt should be made to at least unify the solution across the same "class".
Simplest unified solution imo would be to add a separate tech for these vehicles (the tech could be tied to tech). That would allow to bring timing closer across modes and fix any tech cost issues that might be (with a cost form 0 to what ever is needed).