There's a ton you can do. Especially for free.

You should experiment with 3 different user acquisition channels every month unntil you can eliminate the ones that don't work and basically double down on the channels that's bringing you users. (I have a worksheet for this)

That's what I did for my startups (LawTrades and myFBcoverPhoto) and eventually grew the first to profitability and the latter to 10M hits/month without ever hiring anyone else or paying for users.

I know a lot of people will tell you about content marketing but that's just one peice of the puzzle.

I can't spell out everything for you in this answer but here's a high level overview of some of things we can discuss in a phone call:
- viral marketing (creating an app feedback loop)
- unconventional PR
- meetup.com / community building
- piggy backing exising platforms
- biz dev
- affiliate programs

That's already plenty to keep you busy for the next 6 months but there's more.

I'm a maker on Product Hunt so happy to submit your app there if the fit is right.

I have some free time this week to dig a little deeper if you want to hop on a call.

Either way, don't give up. It won't be busy but if you do the right things consistently it'll defiantly pay off.

Best of luck,
Raad