Assuming T2 is Luchs/Puma/Stuka, what would you want from Battlegroup other than a faster T3 to make it a meaningful choice?
That's a tough question. T2 is, to my understanding, powerful because of how well it synergizes with commanders and because it allows you to not "play fair." You leverage powerful light vehicles to avoid needing to use team weapons, to avoid bleed, and to counter your opponent's team weapons (hence why I called it not playing fair). That puts you ahead in manpower economy and prevents your opponent from establishing themselves on the map - your army becomes super mobile and low bleed. Further, the low bleed facilitates OKW's doctrines. With T1, you would probably have your 4 volks then get an mg34, and end up effectively maxing out your army due to bleed. With t2, you have your 4 volks, get a luchs which critically does not bleed you manpower, then later get a jaeger/falls squad with your accumulated manpower. More well known than that is where you spam light vehicles then get a command panther, I don't think I need to explain why mech is better for this.
The point I'm getting at is that if you're going to go t1 and play a fair, static/defensive game, why even go OKW? T1 OKW is just a worst ostheer with a better early game. At least if you go ostheer, you have a good standard anti tank gun, snipers, easy to use indirect fire, and an excellent HMG. Any design for T1 thats main draw is giving them tools that ostheer has (indirect fire, healing, troop transport and reinforcement - I wouldn't say the flak halftrack has any real draw, design wise) will just make t1 OKW a worse ostheer (or a better ostheer, in which case ostheer gets picked even less).
I don't have anything off of the top of my head that could make T1 attractive yet distinct from T2 and from ostheer, but hope that this can point discussion in a good direction.