So if I am a DL Skymiles Platinum Medallion - set airline to Amex - Book tickets from UK - with high award fees - on Amex - get credit - Wait a month - cancel ? - As simple as that?
If you cancel an award ticket, the taxes will be refunded. I'm not sure if we have DPs but this seems more likely to lead to clawback.
set airline to Amex for all cards in Jan - use a 50$ gift card - book 250$ ticket - refund in 3 days - then cancel in 1 month or so - restart again with next ticket on next card - repeat and rinse?
But this time I get only flight credits on DL for each ticket and not cash back
This would work, but you don't have to start with a gift card -- an existing eCredit or unflown ticket works just as well. If the resulting funds are going to be used by the same passenger you can use $50 GC + credit card #1 to buy ticket 1, then change ticket 1 into ticket 2 and paying the extra amount with CC #2, and so on.