As for the phone, some teens just really aren't glued to them - especially if they cannot use them during school hours anyways. While not common, I would not automatically discount the statement that it was common fer her.
Especially, as it makes it harder for anyone to reach her - such as the abusive stepfather. She might just really have felt better with being out of contact while away from home and mostly used it at home to chat with friends.
One thing that is standing out to me is how quickly they found the CSAM/evidence of sexual abuse. How was it found?
This gives me a tiny hope that maybe the stepfather is not involved in her going missing. Surely he would've known to get rid of such material before trying to pass off "a runaway" story about her? Of course, people are stupid, so who knows. But the LE does see what time he tried to delete stuff - if it was before she was reported as not showing up at school, there is little hope she is alive. If it was after that, however, then maybe something else happened.
It is possible she just tried to escape the situation, but what makes me worried is the "when I turn 13" thing. This is an abstract border. If you want to run away from home because of sexual abuse, you don't really wait until a birthday. It could be connected to a way of transport or even means of accommodation. But my first thought was that this is something someone else told her. Why, who knows. Cold be a grooming thing, as the situation at home made her especially vulnerable. Could even be something the stepfather said - though I find that rather unlikely.
I am not familiar with all the laws in different states, but do minors from age 13 get any 1) rights to travel alone on some transport 2) rights to be out at night alone 3) rights to start (part-time) working 4) age out off child sex abuse lists (even if in age-gap limited situation only) or similar in FL or states nearby?