Those are good points. We dip below freezing but not much, so heaves won't be a problem. What could be a problem is the weight, like you said. I am concerned for shifting. The best thing would be to compact the ground, which I do not plan to do, because well I do not have one, and maybe I could rent one.... I might consider it.

I plan to pour concrete in sections for the floor and press flat smooth stone into it. Which risks of cracking. Lots of research to do. First, I have to remove the pesky tree in the middle of my spot!