I can't quite follow your story here. Not trying to be critical, I just can't follow what's going on. She has "other family" she spends half of her time with? What does that mean? Are you her step-mom, or is your husband (You referred to him as her dad) her step-dad?
I can't tell what is typical... drama? She spends time in her room on her phone? That's pretty normal. She doesn't prefer to be doing "family stuff" and would rather be with her friends? Pretty normal also. Emotional? That *can* be normal/typical. Hormones are going nuts for most kids who are teens. Some years are harder than others.
As for the phone/ipad... does she not understand/have you not made clear to her from the start that these things belong to YOU (her parents) and that she uses them at your discretion? That you can check on them at any time? That there is no "privacy" on those devices, except as you allow, and then it is subject to anything being hacked or on social media (nothing on social media is every truly private)?
I think most teens, at some point, ponder moving out in a moment of anger. Or running away. But in days/years past, it wasn't shared in writing where parents could see it. It was mutterings under their breath as they stomped into their rooms and shut the door.
I guess I just am not following what it is that has you so concerned... one moment you say she is a good student, with good grades, but just before you indicate she is struggling in school. Which is it?
You say you are trying to give her space to self-regulate (what does that even mean?) but "it's gotten worse." WHAT has gotten worse? You don't really say.
Can you give some background, and what has suddenly changed? I just can't tell fro reading your post, I'm sorry.
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I'm re-reading, and after reading the other response you have, are you saying the issue is that she won't unplug from her digital life to spend time with the family?
THAT is the whole thing at issue here? If I am understanding that correctly, then I think it's pretty extreme. I mean, on your part maybe. How old is this teen? I have 2. And we've never really had too many phone/device issues. A few growing pains and being dumb, but not really in terms of them being unable to turn off the phone. We never really expected or demanded that they put their phones out of reach when we do family stuff. Rides in the car? Sure, text your friends. They still are involved in conversations in the car at the same time. Need both hands (to help put the tree on top of the car)? Yes, put the phone in your purse/pocket and lend a hand. But LEAVE IT AT HOME? No. We never expected that.
Leave it in your room while at the dinner table eating? YES. But not just because we are all in the same room.
Remember what it was like being a teen? (I don't know how old you are.) When I was a teen, you got on the phone (with a loooooong cord) and stayed on it with your friends from the time you walked in the door from school. Except for dinner. Then you were back on it until some designated time that was "rude" to call people after (9:00 usually). It's really no different now, except that they aren't attached by a cord, and they don't talk, they text instead. And several of them can be in the same "conversation" at the same time.
Our kids phones are off during Sunday School and church services. As are mine. But they are back on once over. They are expected to behave politely (you don't pull out your phone and start texting when you are having a face to face conversation with someone), but you don't have to leave your phone at home to do that. Teach them how to be polite and use their manners.
If they are not involved in any extra-curricular activities, this can be more difficult for them to master. If they are, their adult supervision usually will require their compliance with no phone use. In Band? ZERO phone use policy during practice. In sports? Left in the locker room or your gym bag. Not on the field, the mat, etc.
It's not unlike teaching kids to have the patience to read versus allowing them to watch TV and video games and never helping them learn to do things at a slower pace. Kids need help to learn these skills. It isn't something you can just tell them and they know how to do it, or even have the ability to do it, without practice. Baby steps, holding the sofa and cruising before walking on their own.
Maybe engage her in physical activity that doesn't allow her to have her phone out. Don't make her put it up, just engage her so that SHE puts it up on her own. Let's go play some basket ball. Or to the beach and swim. Or play volleyball. Or running. Or help you paint something. Or move furniture around. Or .... whatever. Encourage her to do something specific for 20 minutes without her phone... it gets easier to disengage from the phone. It's a skill she'll need as an adult in the working world.
If I've misunderstood the issues involved, please provide more information. Good luck.