Welp, there's your first mistake.
There are ways of locking down phones and apps, I think. I am pretty sure there are apps that will do most of what you want, but they do not have critical mass.
I did set up a Jitsi server for my daughter and her friends at one point when another parent was not keen on allowing kids access to chat and video apps.
You can give kids a basic phone instead of a smartphone.
If I had kids I wouldn't even allow use of a smartphone. I think hardly any BigTech execs let their own kids use these dumpster fires called smartphones and social media. They know there's almost zero benefits to it. Just leads to brain damage, laziness, ADHD, psychological disorders like depression, life-threatening risk-taking, and even su*cicd.
There is Android support for locking things down for kids, but I do not know how effective it is - mine are adults of close to being an adult now.
Its also hard to do without. I would have to pay a lot more for my daughter's bus tickets to get to school if she did not use the bus company app (because that would mean daily tickets instead of monthly which are a lot cheaper). Its where a lot of kids not only discuss things and social, but organise things (although I encourage doing that at a desktop rather than a phone when possible) so kids without get left out.
It is not just sad, it is harmful. "What is life if full of care, we have no time to stand and stare". It is the opposite of mindfulness.