The lawson in the henna is permanently bonded to the proteins in hair you likely have to grow that out, yes it's well known to loosen curl. Henna'd hair needs regular deep treatments afterwards, have you done this? The dryness should not remain, did you use pure body art quality henna or hair dye 'henna' with other ingredients?

When you say a 'chemical' semi permanent do you mean the stuff that is in a conditioner base that would not damage your hair even if you left it twenty four hours or a demi permanent containing alkaline ingredients that will lift the cuticle? Two different animals that are often confused.

I would not ever advocate mixing any chemical demi or permanent with henna, the results are too unpredictable and it's impossible not to get overlap even if you could find a way of perfectly matching the colour. It's not simply a 'crazy colour' result you should be worrying about, some ingredients in box dyes react badly with ingredients in lower quality henna colourants and can destroy your hair. However I think you will struggle to get a semi permanent to take to virgin grey hair.

If you want to go ahead, do plenty of strand tests.
2a-2c, medium texture, porous/ colour treated. Three years CG. Past bra strap length heading for waist.

CO-wash: Inecto coconut/ Elvive Volume Collagen
Treatments: Komaza Care Matani, coconut/ sweet almond oils, Hairveda Sitrinillah
Leave in: Fructis Sleek & Shine (old), Gliss Ultimate Volume, various Elvive
Styler: Umberto Giannini jelly, Boots Essentials gel
Flour sack towel, pixie diffuse or air dry.
Experimenting with: benign neglect