After 6 bleach baths, 1 Colour B4 and 2 toning treatments....

I don't think after all that processing your lengths are ever going to be light enough for pastels. You could do a reverse ombre (I think that's the right name) with the roots lighter fading to a darker color on the lengths. Most bright colours would take on your lengths and the roots are not too far from being light enough to go pastel. You may have to give up on getting completely pastel though, if you have processed your hair that much not got it light enough.

What have you used to tone it? I think it will look much whiter with a very dilute purple dye on it.
What pastel did you want? You could go for a lilac/lavender shade, make it a little darker to cover the yellow areas, and see how it fades. Worst case scenario it will go silver/grey where your hair is darker, which will be a nice neutral base for other pastels.

I used Directions toner both times and I want to go Lilac actually and have no problem with the idea of it going silver/grey.

Is there a particular purple I should go for or does it not matter?

I found it easier to mix my own pastel purples (both to tone blonde hair and for creating actual pastels)with a mix of Neon Blue and Rose Red diluted, that way you can alter the amount of pink or blue to suit your hair. Very blue toned lilacs (like directions Lilac) will possible fade greenish over that base, so you need a little pink in there too.
I am currently using SFX Blue Velvet and Deep Purple heavily diluted, they make the longest lasting pastels. In fact I am actively trying to fade it and it just won't go lol.

I got lilac by using Pravana Violet.

I made a toner for my lengths and used it neat (for 10 mins) in my VERY yellow roots. It came out quite Ombre, very dark and the roots with lots of multi-tonal violet, silver & lilac running through the ends. That was almost 3 weeks ago and now it's faded quite a bit and now it's very lilac at the top (apart from my roots, which are now silver lol) turning silvery white at the ends, still with some deeper purple running through. It's fairly long so it's still predominately lilac, but looks really cool.

Considering it was a complete accident and I did it in a bit of a panic, I really love it. And I'm getting lots of compliments, which is always nice 🙂

Good luck!