I've been running a bootstrapped, lifestyle software business since 2005. I've linked to some articles you might find relevant here (for free!):
https://successfulsoftware.net/starting-a-microisv/
BTW, I have the feeling that nowadays is way harder to live from "traditional" software than some years ago, except for some system software that a SaaS can't replicate, of course.
The downloadable software has been shrinking for years. But there is still a market and all the 'cool kids' are doing SaaS, so less competition.