I don't have that many customers per year, so I just have a directory tree that is of the format <workdir>/<year>/<customer>/<engagement>/ where each engagement

I don't have that many customers per year, so I just have a directory tree that is of the format <workdir>/<year>/<customer>/<engagement>/ where each engagement is a unique identifier for each customer, followed by numbers that represent the YYMMDD of the Statement of Work or whatever sparked off the engagement. Too much more than that would be overkill for my needs. In the <workdir> I have other directories with templates for whatever I need, such as for billing, SOW, payment reminders, etc. Everything is primarily in markdown for simplicity and ease of conversion. I don't use excel, although my invoices once started that way. Eventually I figured excel isn't meant to be a layout platform, and is not compatible with much of anything else in my workflow, and no I longer use it for anything except sorting and visualizing forensic timelines.