How bad is the existing slab? if you use bearers on the slab you have they should even out any rough surface. It's not my area of expertise but I poured a slab last year with my neighbour and we used wooden battens at the sides then poured the slab up to the edge of the batten. I think there's a minimum depth for concrete to stop it from cracking up.
Not terrible tbh, but when I tried to drill it before for an anchor it became apparent it was pretty thin. I think there's some big cracks across the floor, but it seems stable. I would be a little scared that breaking it up would destabilise the remaining walls.
How bad is the existing slab? if you use bearers on the slab you have they should even out any rough surface. It's not my area of expertise but I poured a slab last year with my neighbour and we used wooden battens at the sides then poured the slab up to the edge of the batten. I think there's a minimum depth for concrete to stop it from cracking up.