The most common mistake is studying one subject for a week, then switching to another, and everything just gets jumbled in her head. You can actually prepare properly in six months only if you study all subjects simultaneously, but in small chunks.
Here's what actually works: break each subject not into topics, but into specific task types. For example, in Russian - one day only spelling, another day only punctuation, a third day only essays. That way her brain gets used to one type of work, and she can dig deeper into understanding. Apps don't work because everything's just dumped together, and it's not clear what she's supposed to accomplish that day 😕
As for a tutor - if your daughter can't organize herself, a tutor can help structure exactly that. But only if the tutor isn't just drilling her, but actually teaching her to plan. Otherwise she'll end up with the same problem in university classes later. Maybe start with a few sessions just to map out a roadmap together with someone from outside - sometimes that works better than you convincing.