Well, take my advice with a grain of salt, both because I don't have experience with a child that age and because what I'm about to say may not actually count as advice, but have you considered doing...nothing?
I hear that you are at your wit's end and very frustrated. Although my DS is much younger (almost 2), I can empathize as he is about 90% pee trained and 0% poop trained. If I try even the tiniest bit to suggest where he should poop, he rebels and simply refuses to poop at all. After a few rounds of that, I've completely stopped caring about when or where he poops, and just that he does.
It's one of those areas of control, right? You can't force them to do it. You can't force them to be motivated to do it in the potty. You can try coercive methods like treats, praise, outright bribery, but those don't always work because eventually the shine wears off and they'd rather be the one in control of the situation than get another M&M or sticker.
If it's giving you anxiety, I'm sure she's picking up on that. I wonder if the attention being paid to the matter is only reinforcing the fact that she is the one in control, and that the more you try to coax her, however gently, into doing what you want, the more she will resist.
So, if it were me, I'd do my level best to stop caring. Try to let go of what she *should* be doing. By now she knows what you'd like for her to do, so if you can, try to just leave the rest up to her.
Like I said, my personal experience is that if I try to encourage DS to use the potty for poop, he will simply stop pooping. Since at home he is naked, he has taken to pooping in the back yard. Whatever, man, at least he's pooping!