Are you using permanent, semi permanent or demi permanent colours? Are you saying you have dyed from brown to black to brown? If so have you not tried a peroxide free colour remover?
I think you should be doing strand tests so you know what colour you are likely to get and what condition.
2a-2c, medium texture, porous/ colour treated. Three years CG, growing out mechanical and chemical damage = breakage and very high porosity. Past armpit length heading for waist.
CO-wash: Inecto coconut
Treatments: Komaza Matani, coconut oil, Hairveda Sitrinillah
Leave in: Fructis Sleek & Shine (old), Gliss ultimate volume, Inecto argan
Styler: Umberto Giannini jelly, Boots Essentials gel
Experimenting with: going back to basics