As someone who is also doing a ruined city theme for my bases (albeit mine are
40k, so slightly different), I might be able to help a bit. One way to make cracked flagstones is to get some plasticard and then cut them into several parts, then scrape cracks into them around the shear marks. Small parts of shale can make convincing shards for broken tiles and weathering powders are excellent for making anything look dusty, dirty and tired. I use Forge World's Grey Ash to give my bases that "concrete dust" effect.
EDIT:
Mechanicum Jon has an interesting base style which could be co-opted. Roll out some flagstones for your army (roll to an even thickness, then use a knife to cut them into squares), then, after they have dried, smash them in a controlled way. Put the detritus on your bases, and voila, very effective and convicing broken flagstones.