Update.
Millie has successfully been gaining weight for 6 days now! To say I’m relieved is a total understatement. She is now voraciously eating three hand feedings/day (7-8 hours apart). She mouths pellets and smashes them up and throws them around (lol) but still isn’t really eating them.
The aviator harnesses arrived (I ordered two sizes in case she decides to grow lol). I put it on once a day and take her outside (if it’s nice out, will remain indoors if it’s not). She’s not the biggest fan of the harness. She doesn’t bite at it, which is nice, but she doesn’t really move around either.

Hopefully that comes as she wears it more. I only put it on her for 10-15 minutes at a time right now, go outside, check things out, come back inside and remove it. As she decides to move around I’ll extend how long she wears it making sure to pay attention to if she starts to “get mad” at it lol.
My avian vet told me that we should either keep Millie on a schedule ish and do the same things daily at around the same times or keep her life “chaotic” (lol). and she’ll thrive either way. So I introduce Millie to new things daily and now that her harness has come, we go outside and we go for car rides. We are a “chaotic” family and having her be able to adapt to new things/situations easily is very important to us.
We have started recall training and while I’m sure the only reason she flies to me is because she wants to poop on me (I’m not joking, I think I’ve accidentally trained her to poop on me

) she does come when called and she adores verbal praise (still don’t have a treat she actually eats). I do make sure she’s “in the mood” for flying before I start though to ensure the sessions always have a positive tone. So it’s probably just coincidence that she comes when called, but I’ll take anything I can get lol.
She’s also already imitating us? Which the internet says they don’t do until 8 months or older? She does the “sexy whistle” and seems to do the syllables for “hello” when we say it to her. That could be wishful thinking on my part, but I swear she’s trying! Haha.
Thank you all so much for getting me through (hopefully we’re through it!) her eating aversion and weight loss. Your advice, encouragement, and perspectives truly helped me remain calm and stay the course.
Adding: Millie’s lowest weight was 306g and she is 334g today.