Sorry to keep harping on about the same theme, but BA have caused this with the shoddy implementation of their long haul RFS changes. LHR-SAN will have it’s price in reduced TFS in return for increased Avios amounts. But then adding an AS or AA redemption SAN-SFO will add on the extra Avios for that sector, but also charge full taxes, fees and charges for the primary sector even though still charging RFS Avios. This rip off continues.

Booking the sectors separately will save you a lot but lose your protection if the primary flight is delayed or disrupted.

Just book an AS or AA award for the whole itinerary.