Proper thickness for concrete "flagstones"
Typical pre-cast pavers are about 2&quot; thick, so that's a reasonable experiential datapoint. I've got a few of those in the 16&quot; square size that have been through 11 winters without cracking in a pretty serious winter area.
You can get fibers to mix into your concrete to help reenforce it - 2&quot; is too thin for using steel reenforcement. Ensuring that your mix is not too wet when placed and then kept quite wet for a month after it's set will help ensure you get all the strength your concrete is capable of giving.
You could also pour a 4&quot; steel-reenforced slab on gravel base, and just do patterning/dying on the top surface for the flagstone &quot;effect&quot; - products are sold for this exact purpose (or crews to do so can be hired, preferably after reviewing jobs they have completed.)