Well it all depends on which way the floor is rising or falling. Only you can see that. What ever you do , you need to level the floor and make sure its solid.
If I were sorting it out on the joists I would fix 18mm ply glued and screwed on either side of the joists to bring level. Infill any voids with packers of wood. 18mm ply screwed to the joists ever 200mm. Should be solid then and ready for backerboards etc.