This is why concrete sucks. To keep it from cracking you need a ton of rebar and have it like 6 inches thick. So yeah replace it with a 6 inch slab with tons of rebar.

I use a teak wood bench. Having a built in bench takes up a lot of space

I don’t sand. I wet sponge 🧽. If you use a wide taping knife and a stainless mud pan you can spread the mud nice and thin and taper it out wide. Two coats will do.