Very good tips BUT Carrara will not start a render from where its stoped (only for frames in an animation), If you stop a render this stopped frame will be lost.
Another tip to add here:

For distant objects use low resolution image to map them.

You can compose the final scen in another software like Photoshop, Gimp or... this isnt a bad thing and no one you blame you. All the best images have post production, so dont be afraid to use it!

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