I was like this from about the same age, too.
I would advise against a reward system (for anything) but particularly for something she likes to do that is actively growing her brain cells and neural pathways
I'd second changing the late arvo family routine and bringing forward the wind down time, even if actual bed time remains the same.
Insomnia and chronic readership are not the same, as I did love to sleep, but would rationalise my need for it with the need to satiate my hunger for the story line. She's getting lost in other worlds, and that is going to help her transition to the adult world enormously - though it's really hard to see it that way when you're in the thick of it, as the parent
I don't think it would be a medical issue or even much of a psychological issue, though if it affecting your family and others' sleep, then perhaps a family counsellor could help mediate everyone's needs. I very much doubt a disorder is at play
Helping her develop good sleeping habits is different from enforcing sleep habits. It's not your failure, you just need to work with each other on this one - and that means really factoring in her inclinations, rather than manipulating her to work in with what you want for her.
And this is a billion times better than her being addicted to and sneaking in screen time!