I've been looking at Bryce again in depth. If all you want is to prototype paintings, as you say, and to get a 'base with atmosphere' for overpainting, then Bryce seems the way to go. Also, Bryce interfaces with DAZ Studio via a native bridge plugin, so you get access to all Poser/DAZ content in your runtime. Bryce 7.1's fast Preview render gives a somewhat painterly look, suitable for overpainting. It's no longer free, but only costs $20.

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