I'm obsessed about SDR, because I have seen what happens if nothing is documented. Big thank you for these tips and links.
It is hard to do/find 100% perfect tool/excel for SDR, but to me the real work is to find right kind of process where everyone has 100% commitment for updating SDR. It's not easy on a big organisation and/or when you have multiple people working around tags and especially in the era of TMS where you can so easily populate new variables to the site and documentation is so easily forgetten.
Sometimes I hear comments as "you can always use debugger on the page and see what is populated to Analytics", but that is just wrong way to think. Debugger tells what variables are populated on the page/action, but it doesn't tell what was designed by analyst to happen. There might be errors, missing variables or coder has made a mistake or whatever and that's why SDR document is the single source of truth.
I have some new ideas and comments around this topic, maybe I should update my blog post about SDR or as I call it "Tagging Map".