You can comfortably render 4k or larger and zoom in very close and details will continue to unfold. There are limits and if you get very, very close to the planet, the procedural textures will break down. See the demo video for an example of how close you can get.

Yep. It looks cool too. I recommend setting keyframes on the Mapping nodes of the shaders to animate the coordinates of the textures. This will create a pseudo cloud time-lapse effect, but it will take time and experimentation to get the timing right. Here is an example of textures animated at high speed from an earlier version of the Gas Giant https://vimeo.com/852369602/153f84fc67.

It depends on many factors - computer speed being the main factor. I've tested on two computers running 3000 series Nvidia GPUs, rendering both the cloud and the planet takes 1 - 2 minutes per frame at 1080p. The molecular cloud by itself only takes a few seconds. If you try to render at 4k very close to the planet texture, it could take a lot longer to render.

I have visited many star systems across the galaxy and observed that most gas giants have great spots. No one knows why. You can tinker with the settings to change the size, shape, color, or turn off the great spot if you prefer. Occasionally, other spots may appear in other locations of the planet.