Welcome! Sometimes old hair is just too damaged from past abuse to be salvageable and needs to be cut off, hair is dead so can only be temporarily repaired. Which deep conditioner and leave in are you using?

Are you being SUPER gentle with your twists? I don't really understand the concept of protective styling, it's all manipulation and forcing the hair into an unnatural shape which potentially stresses the hair and causes mechanical damage.

Do you know your hair properties, and have you considered having a hair analysis? Komaza Care are very detailed, will tell you what type of damage you have and how far up the hair it is.
Komaza Care HAIR ANALYSIS Service - Rave! - Long Hair Care Forum
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/ fractionated coconut oils, Hairveda Sitrinillah
Leave in: Fructis Sleek & Shine (old), Gliss Ultimate Volume, various Elvive
Styler: Umberto Giannini jelly, Au Naturale styling gelee
Flour sack towel, pixie diffuse or air dry.
Experimenting with: benign neglect