How do I deal with my boss as an equal?

You can meet authority as one person to another, neither shrinking nor spoiling for a fight, judging each one on the merits. That is a hard-won steadiness. Most people carry an automatic flinch or an automatic bristle toward whoever is in charge. You have set yours down. This does not mean pretending the roles are not real. Your boss does have authority over parts of your work, and that is fine. What you have grasped is that the authority is over the role, not over your worth as a person. From that footing you can disagree without fear and agree without shrinking. Keep it alive by treating each figure of authority as an individual, not a category. Some deserve your respect, some your caution, some your challenge, and you decide which by watching what they actually do. Judge the person, not the position. That is how you stay upright without becoming rigid.