Just curious...natural shades to blonde: where's the 'unattainable' line?

It's not impossible, it just takes a while and a lot of patience. I have brown hair, almost black and i've had white hair and a range of pastels as well.
It sounds like you have a buildup of color so the first thing to do is use color removers to get the permanent colors out. You can use them up to three times in a row. Make sure to condition after as they do dry the hair a bit.
After that, you cannot use bleach/peroxide products for at least a week so use that week to fade more color with anti dandruff shampoo, the bicarb method, etc ( you can find a thread with all the fading methods) . I would then do bleach baths to get the color to a pale blonde. You can do bleach baths once a week( i leave 10 days in between and deep condition after each.) and use a purple/blue shampoo to tone the yellow down.

Pale colours can be achieved over virgin hair, but staining from permanent dyes aren't removable. My ends were dyed black, I used colour removers and bleach to get them to the same level as the rest of my hair before I started the bleaching to pale blonde process. So they have had a lot more chemical processing than the rest of my hair. If I tried to get them white they would break off. They are pale enough for a darkish pastel though, maybe a level 8.5/9. My virgin hair can get to pale blonde with one bleach generally, and it is a darkish brown naturally.
Using a really good bleach makes a huge difference to the amount of lift, I always used to use cheap powder bleach, but won't be buying it again.
Some types of hair are more prone to staining, and some types of hair are too fragile to cope with more than one or two chemical processes, regardless of starting colour.

I feel your pain!  Some hair is just very stubborn.  I have stained hair on my underlayer that will not go any lighter no matter what I do.  It is just mud and I hate it.  I have completely given up on pastel-ish colors(I really don't look good with light hair anyway), but if your roots lighten enough, then just keep up with them and eventually you will have a full head of light blonde hair.  It will just take time.

Most of my natural hair will never bleach below banana peel yellow and most of my dyed hair is already too stained to ever go blonde again.

However I did manage to have nearly white hair (on a few areas) before thanks to a huge purple-blue build up but the fade even further was a weird grayscale shade.

My hair will not shift further than a mid-pale yellow. It's never ever been blonde, it literally just won't go there.

I am pretty sure that bleach maybe with toner will lift even dark virgin hair. However, dark hair that has been stained with dye is a whole different story. I think it may be impossible to bleach pre colored hair as light.

My hair was highlighted before I started bleaching it and my ends just refuse to lift as well. My hair is very long and I wonder if there is still residue from 6-7 years ago and that's why it doesn't lift. (When I was a teen I tried to peroxide my hair and had to box dye over it several times.)

I have medium ash brown hair and it lifts very quickly, I think it not only depends on your starting color, but also your hair's texture.

Thank you guys so much! Yea, I did have a white streak, idk how, and some of the virgin roots got very close to white, but nothing like overall white or the uniform white people get. I could possibly try to continue the roots, BUT that means no dark colors, no natural boxes, and no intense veg dyes. Like I feel like if I put on a full strength pink or purple, I'd just stain them forever and never get it out. Basically I'm wondering if I could do a diluted mid color purple or pink and still be able to get it out later if I need to. I'm not sure I'll ever attempt blonde again, I may just cover over fades with a darker new shade, but it may get muddy so I'm sure I'd have to keep BB occasionally in between (or color remover or whatever) or go 'natural' or 'dark', but once I do that, I feel like it will be the end of my coloring, you know?

Color remover was horrid on me. It turned it blue (it was pinky) and I used it twice. Then I did the bicarb and eventually it faded to a brownish but it never got light like it did with bleach bathing. But that was also not so gentle on me, even leaving a good few *weeks* between BBing and lots of conditioner. I guess length mattters, since my hair is so 'old' but I'm not willing to go short just to get a light color. I do like the idea of pastels, but at the same time, it's not like I must have it. By the time I get it I'd be over it, and a slightly darker version is just as pretty. I was black haired most of my life and finally doing my full head with colors (at this age) is awesome, but I have to hide it for work anyway which is suuuccchhh a bummer and I just hate wearing a wig. 😛 Plus darker is more metal m/ hahaha So I have no idea where I will go from here. I just want to make sure I don't destroy my hair for a specific color. It's not *that* important.