What are the treatments for completely damaged optic nerve due to glaucoma?

None. There is no treatment for end stage glaucoma with severe optic atrophy that has lost all function.
Not many. As of now, with glaucomatous optic nerve damage, there is no way to replace the nerve tissue. Research is progressing in this area but detection and early therapy are the best bets.
End stage. This is tragic when glaucoma progresses to totally damage the optic nerve. This is also equated to severe visual field loss & likely loss of central vision. The treatment approach is similarly to all glaucoma, to lower the intraocular pressure to the range where progression is slowed sufficiently well. At this level of damage most of the treatment strategy would be surgical (incisional or tscpc). .
Notfor better vision. Unfortunately, if the nerve is completely damaged by glaucoma, vision recovery is not possible. It is the cable that carries light to the brain to make images we see. The nerve is essentially an extension of the brain which does not regenerate. Currently, there isn't any way to replace the optic nerve.