I don’t think “bringing up the friends with benefits conversation” is the way to go. Essentially, you’d be asking her to commit to regular casual sex when you haven’t even had sex once. She’d have no idea what she’d be committing herself to, and the only reasonable thing for her to do would be to say “no.”
Every single “friends with benefits” arrangement I’ve had started with a damn good night of sex. Essentially, you hook up, it’s great, and you decide you want to keep doing it. Sometimes that develops into a deeper emotional connection. Sometimes it doesn’t. In any case, sex comes first, then the “friends with benefits” conversation, not the other way around.
That being the case, start asking her on dates and make your sexual interest obvious. Go dancing with her, bowling, fly a kite, etc. On these dates, touch her and kiss her. Don’t be shy about your sexual interest and don’t feel guilty about it. If she rejects you, she rejects you, and she probably will. It sounds like her interest in you is luke-warm, though I suspect that has more to do with your ambiguous behavior for the life of your friendship than a lack of attraction.
If her past behavior’s any indication, she is going to shame/criticize you for your expressions of sexual interest. Maybe she’s uncomfortable with sex in general. Maybe she’s uncomfortable rejecting you. Whatever the reason, you need to uncover it and stop letting her throw up sex-shaming bullshit shields to avoid the conversation.
The best way to address this is to call it out when it happens and reiterate your attraction. When she says something like “you only ever think about sex,” she is essentially saying “there is something wrong with you because you’re showing sexual interest in me,” encouraging you to shame yourself and back off. Call her out on this bullshit. You’ve had a platonic friendship with her for most of your lives and you have way more going on in your life than sex. It’s clear you’re not interested in “just sex” with her. “Obviously not, I’ve got a lot more going on in my life than sex, but I’m pretty damn attracted to you.” Once you’ve called her out, that’s when you show sensitivity. It could be that she thinks you truly are only interested in sex, and once you get it, you’ll leave. You can reassure her that’s not the case. “Just ’cause I’m attracted to you doesn’t mean I’m gonna take off as soon as we hook up. I’ve actually liked our platonic friendship for the past years. I also happen to think you’re sexy as fuck, and now I’m acting on it.”
If she says something like “I don’t want to lose our friendship,” she is implying friendship is incompatible with sex. This is also bullshit, and pretty damn toxic bullshit at that. Call her out on it as well. “And I don’t want to be dishonest about my feelings. I’m attracted to you. If you want to reject me, reject me. If you want to see if we work together, come out with me tonight.” Remember, you aren’t pressuring her to have sex or go out, but you are asking her to make a choice. She must either reject you or go out with you. She has to be honest. If the friendship is something more than your sexual longing for her and her exploitation of it, it can survive the truth that she’s not into you. You will be free to date other women, she should encourage this, and anything you’re doing to gain her affection will be over. Chances are, when she says “I don’t want to lose the friendship,” she actually means “I’m not attracted to you, but I don’t want to lose all of the stuff you do for me because you’re attracted to me.” If being honest about zero sexual interest means the end of the friendship, then you’re not really friends.
If she gives you a clear rejection (which she probably will), date other women. You want to be sleeping with women who are thrilled to fuck you. Anything less is lame.
Thanks for the a2a, and good luck.