I've used 30 volume developer. Plus bleach. My hair is doing just fine. No split ends, breaks or whatever.
I agree with the coconut oil pre-treatment. It really does seem to be protective, to some extent, without interfering with dye uptake. More on that here
Let it absorb for a few hours. It's best if you clarify before applying the coconut oil, so it can penetrate the hair shaft properly.