But I do not want to be a minutemen soldier.
yes and yes just talk to the settlers when you meet them
but you will mos out on one of the best and suprising mission of the game
You can play the MM up to the point of retaking the castle. Once you do that you are a MM and will get alerts.
After returning to Sanctuary and doing Preston's 1st mission. He will ask you to join the MM. Tell him no. Some settlers will still ask for help and allow you to claim workshops.