Your lack of sharpness is not due to the EOS R. It is due to using a longer lens at a slow speed. Look at the stars, they are all elongated. To get sharper with that length of lens, you either have to use a tracking mount or use a faster shutter speed. If you use a faster shutter speed, you can stack images to get more detail. Looking at the image again, it is possible that you bumped the camera or the shutter causes movement. That can be dealt with using a remote release and or shutter lockup.
That or just be happy with the image. If you don't plan to take a lot of astro type photos, do you want to spend hundreds or thousands of dollars to get a little sharper image?