Going from Dirty Blonde to Black

I’ve been dying my hair black (naturally a dirty blonde) for the past 5 years now. I’ve been using 20vol developer on my roots every time which is roughly every 3 months, occasionally to the ends (more often than I would like as my hair has been growing from a buzzcut and has been difficult to part properly).

I find that I have a decent amount of hair breakage, and while it could be caused by other factors this is one I’d like to rule out. I’ve heard that 10vol developer is all I should probably need, but I’d like to know if it would change how the colour performs. If I don’t have to use 20vol then I’d rather use 10vol as it seems less damaging.

I would appreciate any advice, thank you!