It started as just an uptime checker for websites, eventually I added support for APIs and cron jobs, and automated status pages (you may have seen this one yesterday: https://hackernews.onlineornot.com/)
I started it in 2021, I give it two hours a day before work every workday, and I cut scope on most features to ensure they're shippable in two hours. Then I iterate. It works because it's default-alive. I keep a full time job to be able to build it exactly how I want.
Like my React blog, I started it knowing thousands of others were doing the same thing. I made a bet that my unique perspective would be useful to others, and it paid off.
Has been above $500/mo since 2022, growing steadily since (still a few years away from being able to replace my salary).