Term for "professional" who doesn't make their living from that kind of work

"I am a part-time productivity coach."

M-W:

part–time adjective
: involving or working less than customary or standard hours : a part–time job

I "moonlight" as a productivity coach.

• moonlight - (verb) "to work at a second job in addition to your regular job"

• Many people begin their work day when the moon rises. In the nineteenth century, the term "moonlighting" related to thieves who stole at night. However, a July 22, 1957, "Time" magazine article gave the term "moonlighting" a new connotation that has stuck: Working multiple jobs. Having a "moonlighter," or second job, has become prevalent today for economical and professional reasons. The U.S. Bureau of Labor Statistics reports that in September 2013 more than 7 million workers held more than one job. - from http://work.chron.com/

moonlighter - (noun) "a person who holds a second job (usually after hours) TFD

• "Are you interested in becoming a moonlighter? Are you looking for another job to supplement your income? If so, U-Haul is the right place for ..."

• "I'm a dentist by day but I moonlight as a dj."

I like the term "part-time", although some people make a living off of multiple part time jobs. I would use instead:

On the side - in addition to a principal occupation

I am an on-the-side productivity coach. (hyphenated as suggested in the comments)

Or, put the phrase after the noun.

I do productivity coaching on the side, as most of my money comes from elsewhere.