M6 MK II Star shots - dead pixels

My experience with the M6 II for long exposures is similar to what others describe: there are certain pixels that are always 'hot' on longer exposures. There's nothing wrong with your camera.

For night-time city photography, I use the in-camera long-exposure noise reduction which takes a dark frame and subtracts it.

For deep sky work, I don't use the in-camera long exposure noise reduction. As I normally stack images with DeepSkyStacker software, I take 20 or more dark and bias frames at the same ISO just after my session at the same camera temperature, then use those in the stacking process. This should remove the hot pixels and some of the consistent sensor noise etc. Also DeepSkyStacker has specific options to handle hot pixels which you can use.

Incidentally, over on the M6 Mark II Tips thread I learned of a great technique to take multiple exposures for deep sky stacking, dark frames, bias frames etc. Use the M6 II's focus bracketing feature --- it sets the shutter to electronic mode (desirable for vibrationless astrophotos) and when you are in manual focus mode it doesn't alter the focus, but just shoots the exact number of frames you set it for, as fast as it can, without buffering, so it can go indefinitely.

I was using the intervalometer for stacking, but the pain with that is you have to keep figuring out and setting the time between shots to longer than your exposure, or risk it taking fewer shots than you wanted, and you have to turn it back on after each sequence using the menus!

I trigger the camera using the bluetooth remote control app on my phone. With focus bracketing set to 20 exposures, you can fire off 20 dark or bias frames with one press of the shutter, and if you want 100 images for stacking, just push the shutter button five times, or change the focus bracketing number of exposures to 100.

Also as Marco said, I've found that doing 3 or 4 sensor cleans in a row seems to be very effective at removing the hot pixels, at least temporarily. I had some red ones visible in the EVF with the lens cap on that went away forever after a few sensor cleans.