I don't understand all these "mtb gears" and "too short" comments. According to Shimano's website, the R8000 medium cage should handle up to a 34t cog with a chainwrap of up to 39. The OP's setup has a 35 chainwrap, so it should be well within reach. The picture makes me think the chain could be straighter, so take
FBinNY 's advice and pull the chain tighter to see if it will work to take 1" out. If that doesn't work, I'd check the part number to make sure you didn't get a short cage in the wrong box or something.
https://bike.shimano.com/en-EU/produ...-R8000-GS.html