Hi Rosie,
This process is quite simple, if carried out by an experienced colourist!
An option could be to create a dip-dye/Ombre effect using a combination technique of foils and free hand colour - breaking up the colour with soft tones of darker shades helping to blend your regrowth area.
From what you have described I personally would start with a deep conditioning treatment to restore protein and even out the hair’s porosity. Then use a smudging technique at the regrowth to mid-lengths with a dark blonde/light brown formula. To soften the blend and improve the shine, a demi-colour would then be applied in a soft blonde to your older blonde colour. This would require no further lightening at all and would assist in improving your regrowth visibility and condition.
Hope this helps!