Chris Campbell and I have a challenge to see which of us can fire the thickest greenware.
I have various solid clay blocks drying currently, some up to 3 inches thick, with a new custom bisque schedule to fire them.
Alexandre Bigot experimented with very slow-firing kilns for architectural sized ceramics before he switched over to using hydraulic presses to form pieces with grog and other material which did not off-gas.
Regardless of how slow you fire, there is an absolute maximum thickness you can fire with each clay before developing cracks.
Looking at your shards and survivors, I would make a few personal observations. Your quest for answers told you that cold ware that feels damp will have ice in the clay that will hide its amount of wetness, also causing extremes when fired. Secondly I would think you would want the walls of the "rocks" thinner in order to be flottable. The survivors look good, and it would be interesting to see the finished pieces. As I have stated before, I once ran a project with slab construction that used large river stones (5-8#) as forms the students would roll out 3/8" slabs and wrap completely, paddle to smooth, cut in half on a bevel when leather hard and rejoin. You might try the same.