So I tried your j.B > j.6D > 6C string to begin the standard combo, and I was able to get 4714 damage after ending with Gosei.
Afterwards, I tried the Challenge Mode 19 combo (ending with Gekiren) and it looks like it does more damage. (I got 5245 out of it)
[FC] 6B > j.5C(1) > j.B > j.6D > 6C(1) > 236DD > 2D > 236B > j.5B > 5B > 5C(1) > 6C(1) > 236DB > dash > 623C (5245)
You also have the option of ending with Seijyuu after Gekiren for some extra damage if you want, (5905) OR you can activate Overdrive after Hariken instead of doing Gekiren and do OD Seijyuu. (6199) I personally don't think the ~300 extra damage is worth it unless you end up with a long enough OD to replenish Drill after Seijyuu, but it's your call.
Let me know if I did your combo variation wrong though. Was my damage figure about right?