Originally Posted by srode1

The DR isn't designed for anything special on the rear end, it should fit on any frameset that an ultegra DR would work on. The 815 is the same and it will fit on anything. The jumping 2 and then another 2 before the 3rd click doing the front shift may well be a result of the "incompatibility" problem your pop up message indicated. I use a front 8050 and a rear GRX 815 with no issues at all.

Personally, I would start from scratch again and make sure I didn't miss something in the set up starting with the bolting on of the Rear DR to the frame and in the Di2 set up. Ditch all the spacers you have and use the limit screws following the Shimano instructions for set up. The only thing that could have the alignment problem if installed correctly is a bent hanger. Make sure the stop block on the link arm is on the right side of the spot the old "B" screw push point on your DR hanger and that it's right up against it. That won't rotate in place after you tighten down the DR, you need to move it to be in contact before you torque the fastener, but that's the same as the 8050 also.