Pre-flashing will raise the paper inertia point, though getting the right amount without risking safelight fog and veiling the highlights takes some doing. About the only time I use pre-flash is when I am doing safelight tests and want to err on the side of caution!
It is really tedious to do if you only have one enlarger, and you cannot stock pre-flashed paper for very long.
Other options are split-grading - put the highlights in with a soft grade and the shadows with the hard grade - or local application of warm developer on fibre prints. There is also the print heavy and light bleach approach, but it does not sound like you have enough detail to do this.
All these methods can get that little bit of tone in fine highlights, but they do have slightly different effects on the mid-tones and shadows.