The R is a great camera, and I've heard nice things about the A7III as well. I don't have much experience with the astrophotography versions of cameras that Canon sells. My understanding is the UV filter is either pulled or replaced with a different type of filter. Seems like that would make it really excellent if you're just using it for astrophotography, but I'm not sure how it would affect day-to-day photos you take.
Yeah, I'd say if you're doing weddings, go for an R6. The EOS R only has one card slot, so that knocks it out of contention for weddings. That leaves the 5D4 and R6… The 5D4 is a great camera, but at this point it's a better choice to invest in the RF system. The lowish 20 megapixels the R6 offers is a benefit for wedding photographers too. Since many photographers take 1000+ photos per wedding and customers expect a fast turn-around time, the 20mp RAWs are much faster to work with.
