Sunken bathroom problem
Something just not make sense.
If is 6" lower, I first need to question the builder and why?
You can always raise it, but you may find a surprise when you open the floor to see why they did it the way they did.
But yeah, taking the floor out and raising it, I would also raise the tub, you can re use the tub, but would not bury it in the floor.
Just move it while fixing the floor elevation issue .... but I would be looking closer to see why they did it this way in the first place, really could be nothing and just idiots building it ... or may be a actual reason for it.
If it was easy, we would not call it work!