Ummmm.....I don't know if I should comment on bike building again but..... :D
Did you pull the seatpost out, carbon prep the top of the seat tube AND grease the bottom of the post? The bottom of the seatpost rubbing against the inside of the seat tube under pedaling loads is a common source of creaking, and easily missed as not obvious. Most people just carbon prep or grease the top of the seat tube. But long travel droppers sit far down the seat tube and you'll see wear marks on the bottom of the post if not greased.
Just a polite suggestion in case this hadn't been done ;)