Let's start 'em at Large, since the size line places them exactly the same height as an iron golem.
It sounds like they should get improved grab, constrict, rock throwing/rock catching, weapon use (and maybe improvised weapons), and death throes.
Ok, so that suggests this thing is a lot weaker, even though the text seems to imply it's just as strong. I'm inclined to bump to 2d10 like the iron golem. What do you think?
Your reasoning for 2d6 slam makes sense, but it just seems odd considering that they can constrict.
I can support Huge weapons, but I'm not quite getting Swallow Whole, since I think we'd want to restrict to Tiny (which makes it almost pointless).
Overland flight for spelljamming is a nice simplification.
I could see adding a little fire damage somewhere.