And this one, 47 Tuc, the smaller magelenic cloud , and the little yellow one named after my wife , top left.. I had a great couple of days outback, I will post the rest of the galaxy quest as I process them..I hope you like them..
Well I found a camera, not really what I was looking for but so far so good, I found a lowish shutter count D800 for a good price, first impressions were that its a beast, bulky and fairly heavy, but stuck on the back of a 150-600 this actually works and balances well. the other benefits are 36mp and the same battery as the Z, also FX. The autofocus is far quicker than my Z too. Not yet tried it for astro though. It is unknown if it will talk to my asiair through a usb cable, and so far I have not found anyone that has tried it on the asiair mini..Part of it was lenses. Didn't have a wide fast lens and was reluctant to purchase DX specialist lenses. My lenes resulted in sub 20 second shots to avoid star trails and high ISO to get a good exposure (F3.5 and higher). Was a bit noisy.
Camera worked well on bulb and I did some 2 minute star trail shots successfully.
Camera started to go monochromatic for shots longer than 10 seconds (main reason to change cameras). The colour was orange, like sodium vapour street lights. Subtle at first, then went full monochromatic as you took more >10 second shots. Had to do a full factory reset to clear the problem but it would come back. Camera was great for normal shots.
By memory, mirror up was a menu setting and not a dial setting like my D750. Never used mirror up mode on the D5300.
Sounds like your Sigma 150-600 lens is an FX lens. Yes a DX camera will give you the 900mm view, but at lower pixel/mm count (quality). See if you can find a professional photographer who is going mirrorless and buy a high click count FX camera (I did this) and some extension tubes. You could get the same result with a higher quality photo.
Went to full frame (FX) cameras and lenses and I'm having a great time without any problems.