Other tips I can give you:

The shorter missions (5, 10 and 15 minutes) are the ones that pay less individually but it pays a lot to do them several times a day, I recommend doing them 5 to 10 times a day to increase your earnings because they won't take a long time to complete ( after doing the same mission more than 5x the gain per Starbits decreases, in my case doing 10x a day still pays off more than that I gain few Starbits so it doesn't pay to keep doing it). After doing the shortest missions for that amount of times you manage the other missions according to your available time in the day.

From level 15 or when you reach 1000 fans the Ego appears and it decreases the amount of Starbits you earn per mission if your Skill is less than the number of fans you have (Detail: each mission gives you Ego so you always go need more Skill to control it because the percentage of Starbits that decrease per mission is the sum of the amount of Skill you have + the Ego you earn per mission and the Ego is permanent).

There are 2 ways to get more Skill which is through cards and Music Lessons (when you complete a Music Lessons you get Skill, but you don't get Starbits). It doesn't pay much to buy cards that have more Skill than Fans because you get Skills for free so to maximize your gains focus on cards that have more Fans and then you get the difference you need in Music Lessons.

To maximize your skill gains in Musics Lessons you need to buy instruments and they will give you Lucky (there are several types of Music Lessons, always make the one you have the most instruments because you will gain more Skills when you complete them) when you buy cards of instruments buy only 1 type. In my case I only buy Guitars and I have 5000 Lucky with them and that gives me 250~500 Skill per Music Lesson (each lasts 1 hour and consumes 40% energy).

At level 50 you unlock the Starbits Millionaire mission that gives you 10,000 Starbits and lasts 30 minutes (you can only do it once a day), but for that you need to have 1,000,000 Starbits in your wallet to be able to do it.