If you dont mind being a simp or spare tyre if she happens to break up with you, just wait for her

I will disagree with this
she can have a different phone if she wants to cheat, suprise checking will not help, women are better at hiding this kind of stuff

If you need to check her private stuffs, that means you've an expectation that will result in misery. If a woman respects you, there's no need to "tell" her what to do, she will automatically do or assume good behaviors unless if you meet a trashy girl that is born from worse family background and trying to convert her

any form of control is just a wasting effort
you can only control yourself, not the environment - stoicsm