I'm not sure I agree with your assessment regarding getting STGs earlier. The bottleneck for those is munitions not tech. If you are teching at a normal speed, you'll have the munitions for at the very most 2 stg upgrades once BGHQ is built if you didnt bother with any grenades or medkits.
On a more relevant note, I was thinking perhaps locking volks STG upgrade behind BGHQ which would certainly make it picked more, but this is through nerfing mechanized rather through buffing BGHQ and I'm not sure that's a good route to go. Similarly putting Obers in BGHQ would be a simultaneous nerf to Mechanized but buff to BGHQ. I just dont feel mechanized needs a nerf, BGHQ just needs to be more attractive.
Its probably more of a personal experience, as I enjoy munitions play over fuel so i like bee lining them stgs. Take it with a grain of salt. My point however I maintain that bghq should give more to infantry as mech provides vehicles. Make bghq the bread and butter for infantry play.
Perhaps a midway to your suggestion and mine? Increase the cost of stgs by 10 or more and remove it when med is built?
It could be interesting to reintroduce the original "resource starved" okw beck through other means and allow an accommodating power increase as needed.