I believe you can also stop at Wake and just refit. I believe this will generate a new mission. Also, you do not need to keep going to the mission. I have blown off a mission for convoy action

if I come upon them. When I see fat juicy targets, all of a sudden, using my Kodak to take pictures of ships seems secondary to me and my crew. Also, I ran into the repeat mission, I said screw it and went off to find my own thing to do. Eventually, on the same patrol I had blown off the repeat, the finally gave up on the first mission and I was handed another mission.
I look at this way, I'm the Skipper and I take my sub/men were I want. Come back to port with 5-6 sinkings and you are still a hero

. If they want me to go back to the same harbor, they can do it themselves....it's a dangerous mission anyway