My winter project bike is a 2009 vintage Cannondale Street. I’ve got a lot of Di2 components laying around…coincidentally I have a set of ST-R8070 I took off of a bike that was “upgraded” with 11-speed GRX a while back (4 years??), I’ve got an RD-RX805, and I have the XT 11-40 cassette ready to mount on the rear wheel with a DT Swiss hub. As far as I can tell, my only option is to put that RD-RX805 on (or potentially an RD-RX815 from the GRX line), as the XT RD-M8050 won’t function with a road FD-which I need to use due to the FSA SL-K double crankset on the bike.

That's lot of words to ask- how did you get that RD-RX805 to work, just a b-screw adjustment? How about chain wrap, did you use synchro shift to prevent chain combos that the capacity wouldn’t actually allow (such as big-big or small-small)?