Options-
- You can try to use your current RD as 4 teeth beyond stated capacity can still work, but it isnt a certain. If it works, great. If it doesnt, then just get the 105 GS RD.
- If you get a new RD, look into an Ultegra RX RD or a GRX RD. Both are clutched which reduces chain slap. Obviously a 105 RD is fine too. Pair the GS RD to an 11-36 cassette.
- You can just change your 36t inner ring for a 34t inner ring. If its a Shimano crank, those replacement 34t rings should be $15-20 online. That will help give you easier gearing.
- Buy a different crankset- like a 46/30 or any of the other subcompact ones that are 48-46t for the big ring and 30-32 for the small ring.
You cant put an XT RD on because XT MTB RDs dont properly shift with 11sp road shifters due to different pull. You can kludge together a working system by using a Wolftooth TanPan to convert the road pull into the MTB pull. But why do that when you can just use a road GS RD and which can handle an 11-36 cassette?