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If you can find award-seat availability, 150,000 BA Avios would get you close, with no carrier surcharges: MIA-TXL-AUH-TXL-MIA on Air Berlin in Business Class. You would still have to get yourself between AUH and RUH.

Amex occasionally offers transfer bonuses from MR to BAEC Avios, so your 90,000 MR points might yield more than 90,000 BAEC Avios. But don't transfer/convert any of your points until you are sure that the award-seat availability you need exists.

Another option would be to try to amass 60,000 AA miles (perhaps by means of a credit-card sign-up bonus and some spend), and then redeem 100,000 BA Avios for MIA-TXL/DUS-MIA roundtrip in Business on Air Berlin, and then redeem 60,000 AA miles for Europe-RUH-Europe roundtrip in Business. AA's partners that serve RUH include EY, GF, QR, RJ, and 9W; there are no surcharges when redeeming AA miles on any of these partners.