Loving the mahogany - but what to do with my roots?

Hello ladies, I have come to pick brains (again!)

So, I was transitioning from nuclear red over a very light bleached base to a nice rich Adore Mahogany and was having trouble with it fading to a very ashy colour very quickly due to having the light base. Against everybody's better judgement I used Castings (demi) in chili chocolate which did wonders for the lengths (but obv did a bit of damage) but didn't take to my roots much at all (faded out within a week) so I think when my roots need doing I will have to use a brown/mahogany permanent dye then top it up with Adore Mahogany.

I've seen Wicked Pixie mention that she has used a lower vol developer with a box dye before now and I'm thinking that may be my best option but wanted some opinions - my natural colour is light ash brown with blonde bits in and my hair is very resistant to taking colour (very shiny in texture, doesn't hold styles/ backcombing well) and I want it to be maybe a level darker and much warmer. Perhaps a box dye with 20 vol?