I agree with all of the comments, advice and suggestions that you have given.
You are correct in that we have only introduced sketchup this year, actually this term. And the kids absolutely love it although for some the learning curve is steeper than others. Some have downloaded the software at home and their skills are increasing through play.
The reason why I wanted to use a renderer is to show the kids how light and shadow play a part in a model, especially with curved surfaces. Don't get me wrong, I'm planing ahead for most of next year. Most of my students take a while to get used to the tools but they are very engaged in their attempts.
Kerkythea is not right for them but i will look into some of the other options you have given. Please if you come up with any other ideas I would love to hear them.
Thank you