Bricks, Masonry, Asphalt and Concrete - slab on slab considerations

I have an exposed 10' x 15' concrete slab that I would like to leave in place as 17' x 25' new slab is poured over it to match my existing finished floor level for an enclosed patio. The existing slab is composed of several smaller pieces poured over the years. All are sound and at 4" to 6" thick.

I'm concerned that a new slab poured over the old may crack at the edges of the old slab during the curing process or as a result of shifting afterward. The new slab will be at least 2-1/2 inches thick to match my FF as it covers the old. I was planning on matching the bottom edge of the old slab for at least 1-foot past the existing edge then sloping back up to a 4-inch thick slab prior to reaching the 12" x 12" footers.

I was considering using a fiberglass reinforced mix. At this point I'm planning on doing the form-work myself - but may hire out the actual pour and finishing work due to the size unless I can line up several helpers that know what they're doing. I live in Florida, so frost is not a problem. The soil is sand with no clay.

I don't want to waste perfectly good concrete if I don't have to - but I plan on placing a hot tub on the patio and the effect of that weight over or just adjacent to the thin area has me concerned. What say ye?

You will be happier in the long run if you remove the old pieces of concrete and pour a new, continuous slab.