Achieving balance between going lighter, but not losing pigment & warmth

Hi there, great forum, thank you.

I'm a middle aged woman who's slowly going gray. My natural color is light strawberry blonde since childhood, but slowly darkening and losing warmth over time. (my hair color and complexion shade is very similar to Nicole Kidman)

I've been restoring the color using level 9 gold/blonde/red permanent color and touching up with reddish gold demi permanent on the roots.

As my hair keeps getting darker all over - with gray at the roots - I am trying to adjust to cover the grey and keep the warmth.. I want to go blonder to make the transition to grey less noticeable.

My concern is if I bump it up to level 10 or 11 gold blonde - there may not be enough pigment in those formulas to cover the grey - and the rest of my hair is somewhat coarse and resistant.

I am wondering whether I could keep using the level 9 gold red blonde color (since it has a quite a bit of pigment) but bump the developer to 30 or 40? in other words, lift through the developer and not through using less pigment?  I need to keep color!

Any suggestions much appreciated! THANKS