Leveling the tub is important for proper draining, and keeping water from pooling, and maybe end up rotting things because they stay damp.
My whole downstairs was unlevel, and I went the route of stripping all of the subflooring off, planing and shimming the joists, then installing new subflooring. it was a major effort, but with the use of a laser level and time, the whole floor is level to within about 1/8" across the full surface.
If you strip the subflooring off, you could sister new joist segments onto them that are now level, then put new subflooring down on the sisters. this would make installation of the tub, vanity, toilet, and tile, if desired, in the remodel project much easier. the biggest problem comes in a big transition out of the bathroom into the hallway. I redid my upstairs bathroom, and installed pex in a pretty thick bed of slc for radiant heat. this raised the floor easily 1.5", and I don't find the transition to the hallway a problem. I happened to have some stone saddles made that matched the granite counter, but it could also be done with wood or other materials.
If you can live with just the tub level, you could set it in a mortar mix, or choose stuctolite.