first time to bleaching, virgin roots and previously dyed lengths - Suggestions?

Hello

I have previously dyed hair a couple of shades lighter reddish, from my very dark ash blonde natural colour, but now i want to change to bleach and then henna, it is too dark when i go over my natural colour, so i intend to lighten my hair some and i was wondering what the best way to handle the difference between the virgin roots and the lighter lengths to avoid hot roots, and get an even colour throughout. I am going to use a very gentle 20vol ammonia free bleach kit from organic hair systems with the minimum bleach powder i can get away with, all over coconut oil.
I have now taken some pics so it is clearer, this is after 2 vit C treatments to remove colour.

I think my roots are maybe 1-2 shades darker than the length, about 1-2 cm at the moment, not really visible in the pic.
I have previously hennaed my hair from this colour, looked good when first done (pic below), but oxidised to much darker and i was unhappy with the end result, not bright enough, and brownish indoors, so i have pretty much decided on lightening first.

I have done a couple of developer only and developer and baking soda tests on hair from my brush but have not had much luck with results, some hairs seem to lighten and others didnt, and developer only didnt seem to do anything at all. im mostly just a bit nervous about putting that on my hair with no results but still damaging it. Maybe i didnt leave it long enough at 1/2 hr. I was thinking maybe a bleach soap cap might be the gentlest option? not sure.
Sorry about the huge pics.

Any help or suggestions would be great.
Thankyou