Render to Disk will cut down on GUI taxing, so test your image first with a smaller res render, change the image render size(save), and Render to disk (Do not save the file when exiting Bryce). (faster too)

Cut down on reflective, glass, and high-res textured objects.
Volumetric Mats are expensive too.

Save,Save,Save, using altered file names....file001, file002, file003, etc

A starfield can be accomplished using a jpg(made in Universe Image Creator) applied to a sphere or 2D primative with Trans map set.

Also, an alternative to Photoshop is Project Dogwaffle, It's FREE, and from what I've read & seen, is a decent piece of artware.

Good Luck

Oh, btw
If you're still not into post work, you can render a scene with low res materials on your objects, change the materials, and then Spray Render those changes. It can be time consuming though.

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Ockham's razor- It's that simple