It really depends on how much you have to spend but in my experience, the lowest customer acquisition costs come from influencer marketing (non-organic word of mouth). I'm in no way affiliated with them, but Neo Reach (neoreach.com) has a scalable influencer marketing model that is genius. It's pricey, but genius. I can make an intro to one of the co-founders if necessary.

I'd love to help you strategize a plan to increase app downloads. Shoot me a message because I checked out your website and have a few ideas on how you can make some small tweaks to your website and marketing to separate yourselves from your competitors. I'm confident I could be of some help to you as I have a lot of success hacking experience like the situation you're finding yourself in.