EOS R8 Questions on low-light performance, battery life, lens compatibility

[taddy]

I've been using my Canon 6D for a few years and I love it but I'm thinking of making the jump to a mirrorless system. I'm looking at the R8 as it's within my budget and I'm unsure if I want to swap my setup to Sony.

I mostly shoot landscapes and wide-astrophotography so low light performance is essential. The 6D is already exceptional in low light but how much better is the R8?

How do the batteries perform when shooting astro? I know to expect less compared to my 6D - usually I get around 500 long exposures per battery and would like to get more than half of that

Is there any issues using 3rd party lenses with the adapters on canons full frame mirrorless systems?

It's a fair bit of money, so I'm just unsure if it is worth making the move to mirrorless or to just stick with what I already have.

Thanks

[Waddizzle]

I would not recommend it moving from a 6D to an R8.  I really do not know what Canon engineers were thinking with R8 design.  It is a far cry from being a MILC equivalent to a 6D.  

The R8 has the rear panel of a Rebel DSLR.  It does not use the same battery as the 6D.  It uses the same battery found in the most recent Rebel DSLRs.  You would need to invest in a new battery arsenal.

The R8 is more equivalent to a Rebel DSLR with a full frame sensor than it is to the 6D.  Look for an R6 series body.  It costs a bit more.  But you will be jumping for joy with one.  Guaranteed!

[Tronhard]

The closest equivalent to the 6D series is the excellent (IMHO) EOS R6 series (original and MkII) - the similarities of the number are significant. Both are generations ahead of the original 6D, which seems to be what you have.
I personally consider the R6 series  superior to the 6D series for a bunch of factors, not least is a brilliant sensor with far superior dynamic range on both versions: the first is 20MP, the R6MkII is 24.  BTW, the R6II shares that sensor with the R8.

Above is a link to a fairly reasonable comparison video that you may want to view considering the R6, R6II and R8 - just ignore the presenter's style!   Now, given your specific needs, you have no need of In-Body Image Stabilization as you are unlikely to hand-hold a camera to shoot the universe, but the R6 does have the bigger LP-E6NH (the LP-E6 from the 6D will also work) and has the ability to take a battery grip to double your storage.   That said, even if the LP-E17 battery of the R8 has a smaller capacity, one can always swap it out, and there are 3rd-party battery grips available to double that.