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Any reason why a medium cage Ultegra Di2 mech, with either a longer B-screw or a Wolftooth or similar extender, an Ultegra or Dura Ace front mech and a Shimano XT 11-40 cassette wouldn't satisfy your requirements? All Shimano, 34/40 low gear, known to work - the only issue is whether you'd need the extender or whether you could get away with a bottomed-out B screw or longer B screw - and will work on either 10 or 11 speed wheels.
Or use a "Road link" to extend your derailleur hanger.