In my experience, it's really, really tough. To get our first customers we had to send thousands of emails, talk on the phone with dozens of prospective customers (for both customer development and actual selling). There's a reason why SDRs/Lead Gen marketers are full-time positions and it's ambitious to try and replicate a legitimate sales funnel without being able to dedicate time to it.

This isn't something I would have been able to do if I were still employed, which I assume is your situation since you refer to it as "something on the side." For instance, even getting on the phone with people can be tough, since the hours they're more likely to chat with you are the hours you're at work.

I know this isn't something you want to hear. The good news is that I'm sure it depends on the industry - we had zero connections and background in our industry when we started - and I'm sure our experience doesn't reflect _all_ companies. But I admittedly underestimated how much time/effort needed to go into selling a b2b product when I was a comfortably employed developer.

Edit: re-reading your post, I think you might be describing a consumer-facing product. In which case user acquisition is different, but I would say the challenges are the same (having built many consumer-facing side projects in the past).