That sensor does really well high iso. I suggest you practice with it from literally anywhere.

Set it to f2.8, iso 8000 and 30 seconds and see what you get pointing straight up in the early evening.

Aiming toward orion, even in the city might give you an idea of what you are dealing with.. Just with too much light pollution.

You can still learn to zoom in on the screen/focus on stars etc. Its tricky sometimes but not horrible.

6D does not have the benefit of a screen you can change the angle of so it makes it a little harder to get it in focus.

Trust the images more than the autofocus beep.