Assistance with selecting the right equipment for a Sony user

I wouldn't buy such an expensive and heavy zoom lense for astrophotography.  I see a lot of people on here opting for the Samyang 135mm F2 (which weighs less than half the zoom lense) and getting great wide-field results.

For suitable trackers I always point to iOptron Skyguider Pro or Skywatcher StarAdventurer. For the latter you currently can buy the 2i but a more expensive and potentially more capable GTI model is about to get released. For videos on how to use the trackers look for (1) The Narrowband Channel - YouTube or (1) Nebula Photos - YouTube or (1) AstroBackyard - YouTube

Considering the lens the main question in my eyes is whether you buy it primarily for astro photography or primarily for daytime photography. In the first case consider a small scope instead of a lens. LBN 515, LBN 535, LBN 558 ( spacetimepictures ) - AstroBin is a nice example what can be achieved with a tracker + a small refractor + a Sony camera (+ software + a lot of practice to use it properly ...). Alternativly the Samyang 135mm is a nice step up from your current kit, e.g. ngc1499/m45 wide field ( Nenad Vasilijevic ) - AstroBin

Based on your budget you may choose a more expensive Sony or Sigma prime lens which may be more suitable when used for daytime photography too.

Good luck and clear skies
Wolfgang

For suitable trackers I always point to iOptron Skyguider Pro or Skywatcher StarAdventurer. For the latter you currently can buy the 2i but a more expensive and potentially more capable GTI model is about to get released. For videos on how to use the trackers look for (1) The Narrowband Channel - YouTube or (1) Nebula Photos - YouTube or (1) AstroBackyard - YouTube

Considering the lens the main question in my eyes is whether you buy it primarily for astro photography or primarily for daytime photography. In the first case consider a small scope instead of a lens. LBN 515, LBN 535, LBN 558 ( spacetimepictures ) - AstroBin is a nice example what can be achieved with a tracker + a small refractor + a Sony camera (+ software + a lot of practice to use it properly ...). Alternativly the Samyang 135mm is a nice step up from your current kit, e.g. ngc1499/m45 wide field ( Nenad Vasilijevic ) - AstroBin

Based on your budget you may choose a more expensive Sony or Sigma prime lens which may be more suitable when used for daytime photography too.

Good luck and clear skies
Wolfgang

*** Appreciate your detailed feedback, most definitely it would be the latter, where I can have a lens that can do both daytime and Astro photography, from the suggesting I am getting I believe the 600mm might be a hard sell for something like the star tracker 2i or even GTi. 

I may consider either the suggesting Samyang 135 or look into the Sony 135 as well, on the contrary I can also look at 70-200 f2.8 or a 100-400 f4.5-5.6 I a want a more suitable weight/focal length ratio for a 5-600$ star tracker, what do you reckon?***

Personally, I would not try to use my daytime lenses as astrophotography lenses (apart from maybe lunar imaging). 

You could buy an old 1950/60's manual lense like the Carl Zeiss Jena 135mm F3.5 with an adaptor and get used to your tracker before investing in serious glass.  I haven't used one for astro yet, but it takes great everyday photos on an old Sony NEX5N as per the one below: 


FWIW:  the below was taken with a Sony NEX 5N through a pretty cheap Skywatcher 200 PDS:

M31 taken with an old Sony NEX 5N camera

Personally I wouldn't get the 200-600 G for astrophotography *unless* you also wanted that lens for regular photography. There's something to be said for dual-purpose utility. At that focal range, if the primary interest is astrophotography, you'll get much better value buying a telescope. Camera lenses—especially complicated ones such as that—make a lot of compromises to achieve what they achieve and correction across the frame for stars tends not to be a top priority for those optics). Meanwhile, a refractor is purpose-built, a simpler optical design, and can focus all its energy on doing that thing well. Anyway, say, an 80mm refractor, or something like the William Optics RedCat 51 if you're moving into a wider FoV. Also note that something like the 200-600 G or an 80mm refractor becomes a headache and involves compromises on one of those little travel mounts like the SkyGuider Pro (even if you keep under the roughly recommended 1/2 of stated capacity and mount with counter-weights and a rail, the demand on the mount becomes greater as the field of view becomes narrower). Most I'd put on that is something like the RedCat 51 or a less substantial telephoto lens. Since you're buying rather than adapting what you have, don't skimp on the mount.

On the wider end, you've got options like the Rokinon 135, and other options for ultra wide angle shooting such as the Milky Way. And also note that you can adapt popular Canon EF lenses (like the Rokinon SP 14/2.4) to the Sony mount with good results, so you're not just limited to the more restricted and expensive Sony ecosystem.

On the Sony camera make sure you've turned off all noise correction, that you've disabled lens corrections (especially Lens Shading Comp, or whatever silly name they gave it in their menus), and are shooting uncompressed RAW. (Not sure if the A7IV has Sony's new addition of lossless RAW compression as seen in the A1, which might be viable for astrophotography if it is genuinely lossless).