How do you balance between fun dad vs. dad who disciplines?

Most importantly set clear boundaries. Then hold them (and yourself) accountable, let them see your mistakes, show them how to fix their/your mistakes.

I mention the mistakes and fixes because if you only hold them accountable and they don't see you screw up and how you fix it they'll think there is a double standard, making play vs discipline harder for them to handle. At least that is what I've seen with others as my kids were maturing (23 and 16 now). Some parents hid their failures, hid their mistakes but then would hold their kids accountable in a way the kids never saw the parents do for themselves so it causes resentment IME. Then if you play and discipline it feels like a double standard.

I am the goofy, smart ass parent but also the one who holds them the most accountable which, I get, might be odd but it isn't hard to do if you hold yourself accountable as well. Gets easier as they are older and understand more verbally; but you can do this even with very young kids by being dramatic, acting things out essentially, they'll get the emotion on your face & body language even if they don't understand the words yet.

Long ago, before sproutlet was born, I decided that being Father was more important than being a friend. Up to the point that it comes close to being a question, I'm friendly. I've also been quite clear about that boundary. We get along well. 8)