I know that money isn't really the main interest here, but this point should also be noted:
Even if you do get a contract to work in a shared world, your royalties could easily be 2% or lower, even down to zero, where you are only receiving a flat writer's fee.
Compare that to 4-10% on paperbacks and somewhat higher on hardbacks and you can see there is a large potential difference in your ability to survive as a writer.
And to weigh in on the rest - do your own world-building, write in your own world. That itself is practice, because you learn a lot about creating the rules and staying within them. You can practice character, setting, dialogue, etc. by just making up anything - you don't need someone else's world. And when you do need rules of science or magic, you will be better served by beginning to work out the rules and laws of the world you want to write in.
