My children have both had phones for under a year and the condition was I would check them whenever I want and had to know the passcode.
I get what you're saying about privacy, but at this age my two are just learning how to use them
The only social media they have is WhatsApp as I have to approve whatever apps they have. I use family link link others have mentioned. No phones in bedroom at bedtime. They get charged downstairs.
I trust my children but I have seen a couple of messages where one repeated an inappropriate joke he'd heard at school (that's what he told me anyway) and once I explained the context of the joke he was horrified and went to delete it straight away (I already had done that!).
I also saw a conversation where someone else was being quite mean - we talked about how some people might feel braver saying things when it's not face to face and what to do. My older kid brought me his phone once to show me a conversation he was worried about - we chatted it over and it was all sorted.
I said they shouldn't ever type anything they wouldn't be happy with their granny seeing or being on the front page of the local paper.
My kids are ten and thirteen and while they learn to use their phones appropriately I will check them regularly. I see search histories and if things get deleted and I'm suspicious they will lose phone privileges. We had a big talking about sexting too but mine are still "yuck, girls, who'd want to kiss a girl?".
I trust my children but they need to be monitored at this state so they can earn privacy once they've demonstrated responsible use.
I'm saying all this, the ten year olds phone is filled with memes and absolute drivel in some chats, and the older one barely checks some of the groups he is in!