GPU vs CPU render speeds, as to which is fastest, will depend on the scene to be honest.
https://corona-renderer.com/features/proudly-cpu-based remains our take on comparisons between the 2 approaches.
As for 8 GB of GPU RAM, in general that is the most that any card has so the engines should be set up to work with that amount as it is the most common set up. What that means though is it should be able to manage doing any out of core rendering with that amount of VRAM, as complex scenes would likely require more, and having to be out of core does slow things down. Basically though you wouldn't be any worse off than anyone else rendering using GPU :)
For sun and sky, you can use our procedural sun and sky, or use an HDRI (or some or any combination of both).
takes a look at using the sun and sky. You can have multiple suns and skies in a scene to render once and get the results from all of them for use in LightMix.
As for the subscription model, Corona is a little different compared to most - watching many subscription model software, my personal take is that the changes from one version to the next are often not major, resulting in a feeling of frustration at having paid for a year to get very little, and I wouldn't have upgraded if this was a perpetual license. Corona releases are usually significant (apart from the occasional maintenance release where we focus on the code itself, but then those usually appear as an "extra" during the year), so if you were on a perpetual we believe you'd want to upgrade to get all the cool new features, so would have been paying the upgrade price anyway :)
Feel free to give Corona a spin, you can use it for 45 days without any restriction (no render stamps, no resolution limits, you can even use it commercially if you want), but wanted to give you some starter answers too. Hope this helps!