TBH all AA HTs shred light armor to pieces. Quad isn't exception here.
And I would lower its penetration first, it doesn't need to be as effective against lights, but 2 bursts against kubel seem alright.
Yes, but there is an order of things. And I feel like armored cars (222, flaktruck, Puma, etc.) should have and advantage against "MG-trucks". Right now it seems like a soviet truck becomes the ruler of the early-midgame destroying infantry and AC's alike. And it comes in a tier where the other vehicles are actual "tanks". M5 should be a strong AI-only platform in my opinion. With SU76 being the AT option and T70 the middle way.
Your suggestion though goes exactly into my direction of thinking.