For the life of me, I don't understand the disparity in rewards among these missions. It became clear to me early on that even though I enjoyed Fleet Alert and Fleet Defense as much as Colony Invasion, I'd be doing Colony Invasion nonstop. Do I want 22 marks or 31, gee that's a tough one. I remember early on getting -12 marks for Fleet Defense, lol.
Obviously, getting 50 marks in the foundry was salvation. I still played Colony Invasion during the event window, but I didn't have to play EVERY SINGLE WINDOW. I've probably teamed with every player on the server by now, I am that guy that jumps in the village well when we get a perfect score.
Here's a suggestion (and it took all of 5 minutes to come up with, you're welcome Cryptic!):
Every Fleet Event regardless of size (5, 10, 20), or if they are called Fleet Actions, should award a MINIMUM of 25 Fleet Marks regardless of if you succeed with the mission or not. For every completed objective stage, you get an additional 5 Fleet Marks. For the final stage of the event, you get an additional 10 Fleet Marks. For an event such as fending off the invasion fleet at your starbase, you have a minimum 25 marks, 5 marks per wave (8 waves), and an additional 10 marks for the siege group. That comes out to... 75 Fleet Marks. That's a far sight better than the current 20-30 Marks total.
Now, add to this the possibility of an additional 10-20% rewards during "Event" hour, and you got a pretty nice source of income, without making it grindy or exploitive. I'd go so far as to say that every Fleet Event regardless of it's size, could potentially have a 15-30 min cooldown. I'd also suggest a reduction to the cooldown on STFs, to 30 min as well.
Lastly, the Mirror Incursion should also award Fleet Marks, and it should ALWAYS be available. While it's a very nice source of Dilithium, i'd be fine with a 15-30 min cooldown on it, if it's always available.