5" with rebar should be plenty, but also need to make sure you have a good gravel base beneath the concrete. We put a good 8" stonepac base beneath the roughly 5" concrete floor on our new workshop. The stonepac we used is a mix of 3/4" and smaller stones / dust. I'm guessing you don't have that good of a base since the two left squares have a large crack that looks continuous. If the squares are independent with the wood isolating each of the squares, I don't think it would be too likely that the cracks would line up like that unless the base beneath was compromised.