People do seem to say there's just a little bit more banding when pushing a dark photo from the EOS R than the 5D4. I haven't really noticed it though. I think it's only on severely under-exposed photos, like more underexposed than you even want with astrophotography. Also note that the fact that the EOS R's sensor is always active any time the camera is on might lead to a little more heat in the sensor, and therefore a little more noise.
On the other hand, if I'm not mistaken, you can use the silent shutter even in bulb mode on the EOS R, which means absolutely no shake induced by moving parts in the camera while trying to take a long exposure, which is nice.
Honestly it's probably splitting hairs between the 5D4 and the EOS R when it comes to which is better for astrophotography. From a practical standpoint though, the 5D4 is probably a little more rugged and better weather sealed, which is important for a lot of the conditions you'll probably be dealing with when doing astrophotography.