I am a bit biased when it comes to those so-called influencers; I'm not a fan. Simply because they are not all genuine. Yes, they do have a huge following, but that's not what matters. What you should consider is the engagement. If they have one million followers, but only 10 of those engage with their content, then something is fishy. Do your research first before paying them for exposure.
YouTube ads sounds like a good idea. I have never used them, but I have heard they are quite affordable and targeted. I have never worked with any influencers either, so I'm not sure how legit they actually are.