Very good... One idea here to avoid memory errors is to divide the scene in layers:

Background is what is in the... background and no shadow are projected in other objects, so you can render it separeted and add the others layers in front of it.

Foreground is the point of attention in the scene, and can be rendered separeted with alpha.

You can divide in many layers that you think you need and do a post work in PS or others 2D software.

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