Solo-preneurs, how do you launch your products?

I recommend looking at the Business Model Canvas, and getting that sorted out before you write any code. In fact, your early code should be there to validate the risky bits.

Customer channel is one of the bigger challenges. Your first channel should be an organic source. I get 1500 users/month from Bing right now, but that validates search engines like Google as a channel. Your channel might be open source communities, FB groups, gaming forums, whatever. Find an easy way to test that the channel works for you. Then you can branch from that channel into paid marketing (FB ads, events, sponsorships, influencers, etc).

You should be looking for people who have built or pay for a product that is 10x worse. It's probably a spreadsheet in many cases or a mod/plugin. Very often it's someone loudly complaining on a subbreddit somewhere.