Re: Concrete bathroom floor repair
You have one of those super-thick mud job floors. To call it a slab is not exactly accurate. It's resting on dropped sheathing held on cleats attached to the sides of the joist. The tipoff was when you said the tops of the joists were angled.
These kind of mud jobs always crack and have to be replaced. Ideally someone would have told you this before you did any plumbing work, as to repair it and get the necessary support for the new mud job (or whatever) you will need a complete re-plumb.
Remove not the ancient landmark, which your fathers have set.