Before you go to far you need to understand how the floor was build (framed) and what is causing it to sag.
Many plumbers, HVAC installers and the like wreck havoc on a homes construction. If I was quoting on your project and it had a large sag we would be investigating the entier bathroom subfloor and ripping it up to inspect.
The floor joists could be overdrilled. Undersized. Wet.
A main beam could be undersized. Wet. or settling.
If your tub is free standing it may have 4 levelling legs. These would be set to the right height and then the tub lifted out. We like to use Mortar mix 2-3 bags and place some poly down over the sub floor in the center of the tub location. Then the poly back over the mortar mix and then the tub set back in place. Check for level. Then fill the tub with water.
Then secure the tub to the wall studs (Gently).
The poly under the tub makes sure the mortar does not stick to the tub so one day it can be replaced easier. The mortar does not support the tub or level the tub most times only adds mass in the center so the tub feels more solid and does not flex.