How to operate as a freelancer/small dev&design agency?

I recently saw the product newsletterOS that guides people how to build and scale a paid newsletter

I was wondering if any of you solo devs or small teams would be interested in something similar. More specifically a guide with tips on how to find clients (focus on less "cliche"ways) Project Management, Marketing, track financials and profitability etc loaded with different templates?

I have personally founded and scaled software dev agency and currently running another one.