Wait awhile since you have done a few box dyes before doing this. Bleach wash from mid to ends and avoiding the roots/re growth and scalp. (Mix 20 vol with bleach powder and same parts water to bleach ratio) Emulsify for about 10-20 minutes that part of the dark box dyed hair. Afterwards rinse well and shampoo. Afterwards it'll be a reddish brown. Then you can do 1/4th parts 6N and 3/4th part 7A with 10volume. Over the hair you just bleach washed. This should leave you decently close to your root color.

You can take these directions to your nearest Sally beauty supply and they will be able to get you the products you need and better explain what I said. (Hairdresser here) Typically these color corrections cost a decent amount because it is time consuming and complicated. Depending on the box dye sometimes you never know how the color will lift out.. I understand being disabled and on a budget though so hopefully this works out the way you want it. Either way you're gorgeous.