Adding unit doesn't mean that everyone will use it and it will make faction OP. Look at Calliope, look at Pershing, at Mattress... USF from very start wasn't suppoused to have heavy tank - but they got, as option for you in doctrine. Same goes for rocket platforms for UK and USF. An options.
Hummel should be just like that Pershing - an option of OKW player. Feuersturm/Overwatch doctrines doesn't give you too much, way less, than all others. One of them defenitely deserves to be reinforced with Howtizer power.
I actually would add it in Feuersturm, re-name it to "Scorched Earth doctrine" and let it be.
Remember - there is nothing more effective against such hatred blobs, like HOWIITZEEER!
I personally think the game is about making difficult choices. You are right USF had no rocket artillery or heavy tank, Priest was doctrinal from the start. But although Pershing and Calliope was added you can't have all at once. If you go for Pershing you have to quit Priest and Calliope and so on. Same for brits if you go land matress you quit artillery (Sexton), the good heavy tanks (Avre or Crocodile) and elite infantry (Commandos). Either way you will miss something what you may need later on. Your decisions will turn out good or bad, but it is important that you have to decide at all.
If Okw will get its MG34 for supression non-doctrinal there is one of their last unit/mechanic gaps filled. The biggest missing link would be a big (moveable) artillery piece most likely. Since anything else they really need is non-doctrinal there is no real drawback in taking the commander with Hummel, because you would have all unit types available by choosing it. This would be totally different of course if all OKW elite infantry or the KT would be doctrinal for example, but they aren't.
Furthermore its part of the fun in teamgames to complement the unit composition of your mates and to adapt to their strategy. It would be boring if everybody could field any type of unit on his own. So if an axis faction should get the Hummel it definitely should be Ostheer and not OKW. There you would have to quit elite infantry and heavy tanks (Tiger or Elefant) for example for taking Hummel. Thats a choice that I appreciate.