If you are gutting the bathroom remove the subfloor, sister the joists with level joists, then reinstall the subfloor.

If you are not gutting leveling compound or mortar and cement board is the way to go. A skilled tile man can nearly level a floor with varying thinset thickness.

Edit - I’ll echo the “try and identify the cause of sagging first” comments. I have a 100 year old house and things have naturally moved around a bit, but it’s mostly just dips between the joists and things being out of plumb. An entire room dipping so much is pretty sus and it could just happen again after you fix it if the problem is ongoing.