Identity theft concerns about working online

I'd like some input on the topic of identity theft. I've only been working online for a few weeks now, I'm doing freelance writing. Through Upwork, a client asked me for copies of my passport and my W-9 form "for tax purposes." I supplied the information and then realized...maybe that wasn't a good idea? I contacted Upwork and they told me identity verification is allowed during the interview process but, of course, I'm allowed to decline any offer that makes me uncomfortable.

My question is: should I be concerned? Should I purchase a identity monitoring service while freelancing? Does anyone have any experience with this sort of thing?

Thank you!