Sitting in front of a phone all day at this age leads to severe mental health problems and is probably the biggest portal for unfiltered harmful content
You're assuming the relationship is nasty. It isn't a given that people who raise kids differently than you do will fill them with hatred and bitterness.
To me it is an enormous step toward abuse when it is monitoring that the 16 year old doesn't know about. Even for a much younger child I would find that very disturbing. Just doing some filtering or time limits is completely different. I'm over 30 now and if I found out even now that my parents surveilled me secretly when I was 16 I would be disgusted, so just be prepared that your child will never talk to you again if he finds out, if he's anything like me. Maybe he is friendly with you now but it is on false grounds because he doesnt know what you are doing and you are deceiving him
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7012622/