Predictably, this put the Battlegroup/Mechanized right back where it started: Mechanized good, Battlegroup bad.
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As a result, Mechanized is king and Battlegroup is irrelevant.
Given the limited variety in OKW units, Battlegroup can never be made to match up against Mechanized by shuffling around units. Buffing the 251 Flak HT or the ISG won't really change that either, as I don't think it would ever be enough.
Instead, I think it's best to try to indirectly make Battlegroup more attractive by splitting the T4 tech in half and by making the Raketten more reliable (longer range, better durability).
If Obers (and maybe the JP4) can come soon enough, together with a more reliable ATG, it might be able to contest Mechanized and then Battlegroup will indirectly benefit by being more attractive due to its cheaper teching cost.
Putting the Ostwind and Hetzer behind the same "T3.5" tech could make it even more attractive.