How about we find some alternative to him throwing the lightsaber away early - perhaps in their fight on the DS2 wreckage Rey damages it - and then he disappears when he’s thrown down the shaft by Palpatine. Rey defeats Palpatine without him, then leaves. After most other plots have resolved, we show Ben climb back up the shaft, then make his own way out of the wreckage for the final heart-to-heart with Han. So roughly at the same time, as we’re wrapping up all the plotlines, we finally have Rey accept her new legacy, and Ben finally reject his Vader one. We end on the ambiguity of what Ben does next as a free man.