A law is unjust. Break it openly, or change it from within?

Both. You need people doing both at the same time.

Civil disobedience has its place, but you lose public sympathy when you're seen as a lawbreaker instead of a reformer.

This assumes you actually get access to the inside. What if the system actively excludes you? Then breaking it openly becomes the only moral choice.

Change from within respects institutions while still fighting injustice. You can't build a better system if you burn down the whole thing first.

Sometimes breaking an unjust law IS the only way to force change. History shows that.