I am no expert but I have read the same literature as you. My layout will have the same problems as yours.... I will use main bus and sub bus with tail light bulbs.
The way I read it, the 30' limit includes anything attached to that wire including the sub bus and even the track. So any single "line" should stop at 30". BUT there is a cravat. If you use the RF filters then you can go well beyond the 30 feet.
Since I am a suspender/ belt person I will use the RF filter on all bus end and have them run a little beyond the end of the track.
Now others will pop in here and say they have run 50 foot bus with no problems. I understand that but DCC is so fickle there are too many variables to account for all conditions. I am sticking to this plan until experience proves it right or wrong.
Also I plan to build in sections and test the track and wiring with running trains, then move onto longer and longer bus.