Sorry to be the bearer of bad news- crimson crown isnt a 6+. Its worded as a 6.
So stacking +1 to hit doesnt make it a 4+. It would just roll of a 4 for the extra attack.
There are a couple of other items in the BoK book like this too.
They where previously faq'd as being a being synonymous with a 6+. Thats is no longer in the most recent faq for aos2.
I suspect if it does reappear (given how things are worded in the newer battletomes) it will be a natural 6 before modifiers.
However, if it ever does become a 6+ again what about:
Gorepilgrims, WoK BT (crimson crown) BIR (ghyrstrike +1 hit +1 wound).
You could yolo the BIR with a run + charge from the WoKs command ability. Run the WoK to keep crimson crown in range.
Assuming the bloodthirster is in range of your banner and all the 3 slaughter priests make a successful prayer your BIR would be:
5 attacks, hitting on 2s (rr 1s because you charged that turn)
A 2+ to hit gives you an extra attack
So max 10 hits maybe get 8/9 if the dice dont hate you. all wounding on 2s, but each magic 5+ you get does an explosion of 3 mortals to each unit in an 8" bubble. Outrageous carnage indeed!