A single pre 0930 tube journey followed by 4 chargeable bus journeys will cost the price of the tube journey plus the bus cap.
I think you’re conflating two issues. If you start in zone 6 on a train then make lots of zone 1-2 journeys then you will reach the zone 1-6 cap, and that will be corrected by the back office if the zone 6-1 journey plus zones 1-2 cap is less than the zone 1-6 cap. However, where buses are concerned, they will NEVER charge more than the bus cap.