Once the compound reaches a creamy consistency pour a thin layer into the sunken sections then smooth out inconsistencies with a hand trowel.
How to level a wooden bathroom floor.
On the top side of the floor another fix it idea to lay down new hardwood over the existing floor.
You will have to make sure your joists can handle the addition of considerably more weight from the plywood subfloor and any floor coverings.
A plywood subfloor will bridge any minor waves in the existing floor and leveling compound would help too.