Colour B4 and re-oxidation during rinsing stage

I got my hair to a fairly light blonde shade, but then I went through a stressful period with my relationship and on an impulse I dyed it dark brown. Yeah, just a box dye right over the blonde. Naturally, that turned an interesting shade of reddish black-brown on top and a slimy green light brown on my poor, damaged ends. INSTANT REGRET!  *ott*

Thankfully Colour B4 is a thing, and I was in awe when my hair had turned yellow pre-rinsing. However, it reoxidised a LOT - I did expect some, of course, and I was extremely relieved to be rid of the murky dark brown, but it bugged me how most of the darkening happened WHILE I was rinsing. It did darken a tiny bit in the days after too, but as I was rinsing it went from yellowy blonde to a medium reddish brown.

I've done lots of research on how to avoid re-oxidation and honestly, I'm out of ideas. I rinsed for half an hour in scolding hot water, buffering several times and scrunching dry with a towel in the middle of it - I doubt the rinsing is the problem, both because I've done it perfectly by the book, but also because my hair had severely darkened before I even dried my hair mid-rinse.

I was initially happy with my reddish color but now the brassy tones are starting to get on my nerves and I really miss my blonde. I know that with my hair re-oxidising a lot I can't expect it to be as light as it was, but I want it to be some kind of blonde, at least. Does anyone have any ideas? Should I crank up the water pressure? Rinse for longer with just water before buffering, or buffering right away? I know to use it on clean, non-conditioned hair.