Dyeing Hair Back to More Natural Color?

In a stupid moment I put my hair in a pony and cut off about 6". The hair on top is 100% white while the back is 25% gray. I have been coloring it dark brown since 20s when the top turned white. Now without all that hair I see that the top is actually a dark ashy brown while the back is med auburn/mahogany which is probably closer to my long ago natural color. It looks like I have ombre hair. I can't afford to go to a pro every 3 weeks for the roots, so I do it myself. I colored the roots once since the cut because they were really white, but used a semi perm #5 brown whereas in the past I always used #4 brown.

My question is, I need to do the roots again and want to go back to a permanent color, but closer to the med auburn, preferably mahogany. I am not sure, but I think I should find the color, do all roots then in the last 10 minutes pull through back only, leaving the really dark brown on top alone. I think that while it's shorter I'll get rid of that dark color by trims and layers. Any advice would be greatly appreciated as I'm a bit lost.

I went into my salon and all 3 stylists gave me different solutions. Mostly they recommended adding highlights/lowlights to the top which I don't want to do. Is my idea the best route to go or should I try and lift some of the long accumulated dark color? Thx so much! My hair type is wavy, thick, and very course on top. There is no breakage or other damage. The previous perm color was Loreal Dark Ash Brown.