All is know about henna is that perm is more damaging than henna, but I had a dreadful result with henna so I never tried it again. Your idea about low lights sounds promising! It would give a dimensional look to the color.

If you decide to use a perm. color to cover the grays, consider buying two boxes and using them both at the same time, alternating. Streak one part with one color and the next with the other. It gives a much more realistic appearance with highlights and shading and avoids the one color all over flat look. I like L'oreal Preference.

Good luck - let us know how it turns out!
2/c and some 3A. Modified CG.
Protein sensitive
Highly porous. Color over grey.
Best 1st day method: Super Soaker
Conditioners: Curl Junkie Beauticurls Strengthening Conditioner,
Deep condish: Curl Junkie Curl Rehab
Stylers: Mix Curls in a Bottle into everything for shine. Terrible pj
Sometimes try roller sets - classic glamor but I prefer my curls.
Every day is a gift