I've been lucky enough to be independent for 10 years thanks to a web-app (Oxbridge Notes)

Along the way, I learned a lot concerning both system design (e.g. dependencies, CI testing, monitoring) and also the marketing side required to channel enough customers into the business to generate revenue.

Lately I've recorded 25 or so videos about these topics — basically a day in the life in small tech. If this is your world (or if you plan for this to be your world), these videos should have value to you: https://www.semicolonandsons.com/