Bleaching roots?

This is probably a silly question to ask, also i have looked to see if there is anything similar that has been asked before but i couldn't find anything

I have only been using veggie dyes for around 2 months (although i dabbled in them in my teens) and i am a little bit confused about root regrowth.... I am using Directions rose red on lightened hair & each time i have applied it i have bleach bathed my whole head in an attempt to remove some slight staining i have on my ends which becomes visible when it starts to fade slightly.

The last time i bleach bathed my hair i conditioned the hell out of it because it felt like it needed it, then the following day i coloured it, but it didn't take because i forgot to wash it before hand so the conditioner from the day before acted as a barrier (i also accidentally diluted it with shampoo rather than conditioner) It came out a kind of pink to peach ombre which i left until it all faded out (about 2 weeks) then i correctly applied rose red and thats what i have now.

Except i have a 2 week old application of rose red on my hair which is still really bright but roots with nearly a months worth of growth... in an ideal world i would try to put up with this until its faded a bit more, but i have an interview on Friday and i am going to Glastonbury festival on Monday so i would love to have freshly dyed hair for both of these (more so the interview)

I have read up on the fading techniques but these seem to be mainly for if you want to change your colour completely & i just want to do my roots, but avoid what i have done so far in bleach bathing my whole head as the reason i decided on veggie dyes is because you don't have to process your hair each time you dye it.

So i guess what i am getting at, is if i bleach my roots and apply rose red to the whole of my hair, will the colour be even (i think rose red is quite forgiving, but thats a quite a difference in colour, i am a mousey brown naturally) also whats the best way to do this because i have psoriasis on my scalp on the back of my head so if i bleach my roots at the back first and work my way to the front it starts to sting quite quickly, and the amount of time it takes me (i have thick hair) to get from front to back or back to front means the bits which were applied first are ready before I've even finished applying. Does anyone ever do their roots in two sessions to avoid over processing?