At one time in my life, when I was still in love with Emacs, I was using org-mode to do that kind of things as it wasn't breaking my workflow. The thing is tha

At one time in my life, when I was still in love with Emacs, I was using org-mode to do that kind of things as it wasn't breaking my workflow. The thing is that you're a single developer, so don't you waste time managing customers and choose the simplest tool which gets the job done. I know that others are saying Asana or Trello - and those are good tools, but with a bit of customisation and 20$ you might get yourself Jira + Confluence - those will work like a charm and you will be able to not only manage customers, but also projects and all that crap, everything in one place. One often overlooked fact is that customers like to ignore all those sexy markdown files with requirements and they don't care about code versioning - you will be receiving PowerPoint presentations and Word documents and Excel spreadsheets - all of which can be easily managed / kept in Confluence. Remember - CRMs are for business people and whatever you choose, the time spent using it should be as minimal as possible as it is not directly bringing you any revenue.