I'm not sure if my advice is correct but if you want me to give advice based on my experience. I unlocked all Band slots in 12 missions. Focus on unlocking 1 slot per day, if a slot is unlocked, don't try again that day.
I would also like to point out that do not rush to unlock these slots. There's not much use in unlocking these locks before you reach level 85. I could even wait until level 100. Because there is not a huge difference in earnings.
But of course, unlocking is important for experience and more option. You just don't need to rush.
I always start the Local Mini Tour Support mission before I go to bed at night since it takes the longest.