>>4493860AF SLRs are honestly cheating. Extra cheating for Canon because you can slap modern stabilised lenses with excellent abberation control onto a 30 year old camera and get results you couldnt dream of at the time. Excellent/10.
>>4493870All the cameras you have listed are basically top tier and generally overkill unless money is no object. The only incorrect choice is the Snoy.
24MP is absolutely fine for full frame cameras. As you go higher in MP your lenses need to have even more resolution and corrections to avoid showing the stronger abberations that lower MP cameras don't need. Extra MP is absolutely great, but just keep in mind there's limited gain to be had unless you're buying top shelf lenses as well.
I recommend saving a couple grand and getting something used. Used R6IIs are going for like $1500 now because of the III release.
Use the extra money you save for better lenses and accessories. A $1000 star tracker is going to make much greater improvements to your astrophotography than spending another grand on the fanciest camera body.
With RF Canon bodies you can also buy the EF adapter and slap some very very nice EF primes on your camera with zero downsides. The EF 200mm f/2.8 can be had for like $400-500 and will destroy your dick and balls with how good it can be paired with a star tracker.
https://www.absolutelynothing.co.uk/blog/canon-200mm-28-astrophotography-review 