"Identity not behaviour" is a good way of putting it and also my understanding of asexuality and others. (I should note, though, that I'm not speaking from an "own voices" perspective here.)
Oh, I think bringing the themes out even more could absolutely help here. What might help is to really illustrate the shift D/A's thinking.
When I first read the prologue, for example, I didn't get the impression that she was mostly running away to find R; I had the impression that she was running away from things being generally bad, and finding R and/or her family was basically a bonus. All of those things can still be true, of course, but it might help to shift the focus of those first scenes to have her more focused on R. Then it will mean more when she starts "falling out of love" - she gets caught up in all these other things and R starts to become less important. Eventually, we should probably see her making choices that feel more deliberate about choosing things other than R - maybe she chooses her own safety over R, finding her mom and sister over finding R, her work over finding R, etc - but making it more apparent that she's deliberately choosing other things. She can have feelings about that: maybe she's conflicted about choosing one over the other, maybe she realizes that she hasn't thought about R in a while and has feelings about that, etc. (Which doesn't need to be long inner monologues: the Emotion Thesaurus kinds of sentences you pointed out could go a long way towards illustrating how she feels about the choices she makes.) Plus, if R is more of a presence in A's mind before she appears on the page, we'll be more prepared for that appearance and that will also help the story be more focused.
Ah, I did not know this about her!
Especially if this history comes more into play in the sequel, your instinct to save the long explanation for the sequel might be correct; there's the potential pacing issue but also just kind of a lot of things happening. But, I still think this is important information for us to have about the protagonist as it does of course shape her thinking, so maybe there are smaller snippets of it we can get here and there, while this longer segment appears in the second book especially if this is where the history has more direct bearing on the book.