The concrete floor sank, probably decades ago from poor prep. Has since been stable. Was poured 30+ years ago

There is no heaving as it has rebar. It is not painted and the surface is in great condition - just sunk down ~3" over a 15 ft span.

I have seen videos and read about the self-leveling material you float onto the floor and feather in, but I have also heard it is very 'soft' compared to concrete.

Anyone here have any experience with this?