10,000?? I don't think so. A5 range at full payload:A5 itself is 50tonne missile. It is already 10000km range if we go by the standards of similar missiles by weight. I don't think there is a need for A6 at all. The same missile with modified 3rd stage is all it takes for MIRV or any new addition
5000 km with MIRVs
6000 km with unitary warhead
While 8000 km at minimum payload.