You need to check out the correct settings as the flash sync speed for you 6D ii is 1/180th which you should be able to use at 1/200th so only being able to get less than 1/160th would indicate incorrect setting in the external flash mode or a slight fault.
Hmmm... if the viewfinder exposure bar is also out of focus, I think the problem might be the manual focus for the viewfinder itself. Usually cameras have little thumbscrew somewhere near the viewfinder. It's there to accommodate differences in some peoples vision (do they have their glasses on or not ?). Yup... that's my bet. Give it a go. Is it on the top, right of the viewfinder?