"If you make a stopover, the bag charge kicks in"
Never been the case with us.
We fly from Sydney to our destination in the mid west and then later do a couple of separate domestic legs before flying home.
Never been charged the baggage fee.
Our latest October itinerary has the separate domestic legs (4 of them) listed as no charge.
Am flying with my partner who has a chase united card.
Maybe that makes a difference.