Unfortunately you cannot mix reward and paid tickets, under the same booking

It will need to be a separate booking, with its own PNR/booking reference.

If an additional reward seat is released, you could add that to your original booking. That would perhaps suggest that you might want to wait for as long as you reasonably can prior to purchasing a cash ticket, but not so long that cash prices start to rise due to being close in.

I'll defer to others on the best time to buy, perhaps set up a monitor of the cost on one of the sites that offer that, holding off until it starts to rise?

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