It doesn't sound dull to me. The stories are about the periphery of the Culture because that gets the most storytelling value out of the effort that went into worldbuilding, not because it would be impossible to write interesting stories about ordinary Culture members. I don't think you need external threats to give life meaning. Look at the popularity of sports in real life. The challenge there is self-imposed, but people still care greatly about who wins.

When your hobbies can include things like jumping off mountains without parachutes? That's boring only for the people who secretly dream of being a spy.