Thanks again to everyone who posted, it gave me so many ideas and so much to think about!
I thought about how to frame what's challenging me, and it's specifically that this character has to maintain her plot functions as not-raging and lacking physical agency, while not falling into the trap of being a cliched female accessory - and that's what was bothering me about her, and why she didn't feel complete. I couldn't completely put my finger on what was wrong, but I knew it was there. She has plot points that deepen some aspects of her inner conflict, but they don't happen until mid-late book 1, and again at mid-late series, and I felt like she wasn't generating enough intrigue until then.
So what I decided to do - give her more control of information. She has a secret that I originally planned to spill early, but she can actually keep it until about halfway through the book and it will make things way more intense.
I like the running away from home idea. She's half human/half not. I was originally going to have her from the human side, searching for a way to get to the other side. But if I make her from the other side, and start the story after she's already escaped into the human side makes her a lot more interesting, capable, and mysterious. It establishes her agency.
(Also, concerning demon sex - it is with the hero!
Thought I should mention that. )
I don't have a problem with ALL female characters, but so many of the insights here are valuable and were completely worth pondering, because it's crazy how much of the outside world anyone can absorb unconsciously.
I do have lots of awesome ladies in my cast -
The church is run by women (which has more power than the king). The head of the church does some Bad Stuff, but it's because she's secretly in battle with Bigger Bad (which makes her Totally Badass), and she's an enemy that turns tenuous ally at a later point. My marshal is a woman. Another woman who refuses to remarry when expected is the reason that a lot goes wrong in the first place. I have a sorceress who is ultra wicked, but will do anything for her daughter (I loved Atia's character in the Rome series and drew some inspiration there). My tertiary characters are an even split of men/women. I have a magitech realm where women run most of the politics (nobody gets there in the first book, but it's coming). I have LGBTQ characters. So it's not like stereotypical patriarchal characters are the ONLY ones who get to do anything interesting.
I do have a POV character who is a misogynist scumbag, but if I can't establish him as unreliable and a complete scumbag (gross misogyny is only one of his revolting characteristics), then I've failed as an author.
One of my goals is to juxtapose enough wildly different viewpoints in an attempt to make the reader think.
Just this goodie-goodie FMC was killing me. But more magic, more cunning, more knowledge, and more withholding of information is precisely what I need to spruce her up.
And also: so many good thoughts about Lawful Good. Judge Dredd is one of my favorite movies, and I'm having a blast with Tales of Berseria at the moment. Light doesn't always equal good. And a sincerely sanctimonious jerk would be fun to write. I toyed with the idea for FMC and while I don't think it will work for her, it's cooking in the back of my brain somewhere.