Seven years ago I solo-started an automatic time tracker for programmers called WakaTime [1] and launched here on HN [2]. Partly from listening to developers too much, I waited way too long (almost a year) before adding a paid plan, but now it generates more MRR than an SF developer salary not including stock options. Technically I make more from RSUs and stock from past startups as a regular employee, but if I wasn't lucky with those then it would be my highest income stream.
1: https://wakatime.com/about
2: https://news.ycombinator.com/item?id=6046227
For anyone thinking it's egregiously difficult to start a solo-project: You're right, but if you stick with it your persistence will pay off. For solo-products, I think grit is the deciding factor between success and failure.