The time old question - agents or editors?
If you truly want this as a career, many people will say to focus on finding an agent. They are the ones who can get you into the bigger houses that most writers don't have access to, plus target your manuscript to the editors that they think is the right fit. However, you are correct. For PB writers, most agents want to see 2 - 3 polished manuscripts as they want to feel comfortable that you are capable of more than one book and feel they can sell the style you write in. It's hard to tell that from just one PB. BUT, for most agent submissions, you still start out with submitting one PB. If they like it, they'll ask to see more.
Some PB writers have been extremely successful without ever having an agent. But, that will mean that your opportunities to submit to the bigger houses are limited as they usually only give you access if you attend a conference that they are faculty at. But while the number isn't huge, there are houses that do accept unagented (unsolicited) submissions.
And in response to your question about "Would querying agents be a waste of time since I haven't published any PB. . ." That is not an issue. Agents care about the writing and if they think they can sell it. The realize that authors have to start somewhere. Many, many unpublished authors find agents. I am one of them. I queried for several years as an unpublished author and I did land an agent. And my debut is coming out next spring. :-)