We had a similar situation. Children's social care were involved , and CAMHS for my eldest. My eldest does have additional needs and serious mental illness.
The crisis reached a head in 2016/7, so perhaps services were not quite as stretched then. It reached the point of Child Protection Plans, because the eldest son's behaviour was affecting his brother, and my youngest son's retaliation (they were closer in age - 17 and 14/5) was affecting the eldest. Both boys were unable to attend school at this time (extreme anxiety and other MH needs). This made other services get involved, and it also made the situation almost impossible to manage at home.
At 18, my eldest moved into a residential home, for people with mental health difficulties. It was local and we met up very regularly and spoke daily. He is now living in a supported flat and is making steps toward independence.
Every situation is different, and I am skimming over a very complex situation. However, important factors which led to my eldest moving out of the family home are these.
My eldest son needed and wanted to move away from home but services are so hard to find for a 17 year old (this is a national problem). This is why we had to wait until he was 18.
Children's social care were involved at the highest level because both sons were in danger.
The residential unit was very close to where we lived, and had an open door policy. My eldest was able to get the support he needed, and to have lots of contact at home.
My youngest son's mental health suffered.
The effect that challenging behaviour has on a family and on individuals in the family cannot be underestimated.