"To be as safe as possible using the air on the bus, we tap in after the bus service air but before the e-brake air, if that makes sense."

No it does not make sense. But that doesn't mean you are incorrect, it just means I still haven't grasped it. I always treated the air supply to the emergency brakes which is "on" while in motion as a totally separate system from service brake air.

I would have guessed you would tap in at the relay valve to the tag axle. I am still unclear how the minimal air flow to your system would have a measurable impact on the timing between the front and rear.