A protein treatment could be a good idea, as is a deep conditioning treatment.
There are a lot of CG gels you could try rather than using one with cones in it. If you are using no sulfate cleaners on your hair, the cones in a gel could build up and cause more problems for you. LA Looks and Eco Styler are two that are used a lot around here.
Dying from blond back to brown can turn your hair green, so if you do this, I recommend getting it done professionally. When I did this, many years ago, I had better luck going red first, then brown, but since your hair is already badly damaged, that might not be a good idea.