"Quiero escribir más en mi trabajo."
Translation:I want to write more at my job.
There is not much distinction between 'work' or 'job'; "empleo" is 'job', 'use' and 'employment' as well.
To do something ¨on the job¨is an idiom and it is not generally used for specific jobs or people, eg ¨he writes a lot on the job¨ but ¨he writes a lot for/ in / at HIS job.¨
You would need include the word WHILE for that sentence to make any sense.
I think a better translation would be : "I want to write more in my work." The person wants to have writing be a bigger part of their duties in their profession, no just in the office.
No, you tend to use it more. That doesn't mean "we" use it more. "At my job", "for my job", much more common where i am from than "in my job". Usually when we use "in" here, it is for a specific type of position.