I was looking for the same a while ago on HN. Below is the thread. Not specific to solopreneurs, but a lot of relevant advice, including top comment by a solopreneur with 3 successful businesses.

https://news.ycombinator.com/item?id=41862332

Interesting to see spreadsheet in some answers.

We used Pabbly automations on our Google Sheets to send WhatsApp messages to candidates, fetch candidates confirmation for scheduled interview in Sheets directly, etc.

We used HRflow.ai API to fetch information into spreadhsheet directly through add-on

You constitute over 90 percent of all businesses, and you shut down or stay small over 70 percent of the time.

I am adding more features to my Google Sheets add-on as requested by users. I planned to keep it a micro-SaaS but it seems people want it all at one place. They will pay or not is another thing.

https://workspace.google.com/marketplace/app/hiretale_ats/38...

Need to keep annoyance balanced to avoid getting flagged/downvoted though.

Target audience?

Is there a common theme in their startups, or is it working for all types?

> Well-regulated breathing is essential to our health in all respects.

Absolutely. Including mental and spiritual health.

Startup idea to unlock phone/PC by a number of deep breaths as password?

It's good to see these success stories, but is also important to account for survivor bias before blindly following their rule book.