I'm no expert but once I tried just 30 vol. peroxide for about 30 minutes: very small change.
Then I tried a mixture of shampoo + the bleach mixture: again, barely a change, but better. Also for 30 minutes.
5 minutes with bleach powder and 20 vol. peroxide lightens my hair to yellow blonde (worked with level 5 light brown dye or 2 applications of henna), so that's what I choose to do.
I don't have any damage, but my hair lightens fast.