I don't actually think the Flak is bad for it's timing at the moment but I feel like Battlegroup HQ is too expensive in general at the moment. If you think about it, it's 70 mp 15 fuel for the truck, 200 mp and 25 fuel to drop Battlegroup and 100 mp 15 fuel for medics. That ends up being 370 mp and 55 fuel just for healing access. This delays the Flak Halftrack heavily in my opinion, if the Battlegroup was lets say 150 manpower instead of 200 and the truck came with the medic upgrade you could save 15 fuel and 150 manpower which could help you tech quicker getting Obers out faster or being able to rush the Flak HT without having to skip out on healing. The Flak comes to late if you get medics first and the rest of the truck doesn't really help with the Retreat Point being overpriced. ISG's are nice but when you could have the Luchs and the Puma you would take them instead. Mech at the moment is essentially 270 mp 60 fuel, meaning you spend 5 more fuel but you save 100 manpower which is more noticeable. Mech is generally cheaper and has better units resulting in more people going for it, the truck only becomes more expensive when you spend 100 mp 15 fuel on the mechanics but people don't tend to do this in the early game since repairs aren't as big of an issue.
You could just not buy medics right away and get the Flak first while surviving on medcrates as it is the current meta. I think OKWs fuel and tech timing is pretty balanced compared to other factions, so I would not give them an upgrade for free unless you you put the cost somewhere else.
In my eyes the Battlegroup is a good building. Can give you medics, front line reinforcement and you tech up faster than with T2.
For me it clearly comes down to the units and the current LV meta. Let's face it, probably no other building of any faction has such useful units as OKW T2. Luchs, Puma, StuKa - they're all great. But they're not really unbalanced, all prices and timings are alright. So how does Battlegroup compete woth that? IR halftrack - only useful in larger team games. ISG - decent, but OKW rarely needs early game AI power, especially not against in general more mobile Allied infantry. And the Flak - that's being discussed here, in my eyes it does not add enough value to the building.so what's there to do?
Buff by price reduction - could mess up overall OW timings and game balance.
Put Luchs or Puma into Battlegroup - would be an overall berf to OKW
Buff Battlegroup units to give them more shock value (ISG is standardized, IR halftrack has no combat power, so Flak remains)