It's great you're using a harness. We have a nice garden, and we wanted to be able to take Tiki out with us and show her the pretty flowers. So, we bought a harness, too, and we would re-introduce it to her, and put it on her, so she could become comfortable with it. She would bite the harness as it was on her, but the shop owners helped us by putting it on her, too.
That's cute about the cashews.

Hopefully your bird will realize, "Hey! This thing she wants to put on me means I get to go
outside and get those tasty nuts!"

Keep us updated on your progress with your bird.