An easier solution is plasterboard already bonded to PIR or XPS. The XPS is cheaper and I don't think it will make a massive difference depending on all the other challenges like windows and doors. You can simply stick this to existing walls with adhesive foam and then dry-line or skim the boards afterwards.
To do it well, though, you will often need to cut out/remove any laminate flooring so the boards can meet the floor and they won't do anything about leakage between floors, which can be significant.
As you point out they are normally ~25mm or so of EPS/XPS. which doesn't do _much_, especially compared to 600mm of stone. Plus XPS is pretty flammable, so with electricity cables sandwiched between it, its not a great thing to have inside the house.
To do it well, though, you will often need to cut out/remove any laminate flooring so the boards can meet the floor and they won't do anything about leakage between floors, which can be significant.