remember you have 14 days to initiate your return to apple!
you might get lucky and see stock in to reserve show online form 6am Friday. if you find the Pro Max model you want, then you can reserve and collect as you say.
you could also try your luck in a store and see what they have available on the day you try a return.
so if you buy the pro max on a card, you can then get your pro refunded back to apple vouchers, and then get the pro max refunded back to your card and then buy again with vouchers! remember the refund to credit card will take a number of working days to show up on your bill.... If your statement gets issued in that time, you might find yourself out of pocket for a bit..
the other option is to try use the IUP stock checker:
https://reserve-prime.apple.com/GB/en_GB/reserve/A/select?iUP=E
if you can find stock in a store, it reserves one without taking payment (as you have to sign up to IUP in store). you 'could' turn up for that IUP reservation and then just buy outright too!