Getting Things Done is a book that describes a system for... Well you get it. There doesn't have to be anything commercial about it, you can find synopses online for free and the system is simple so those are probably nearly as good as reading the book.

I read it about ten years ago and still use some parts of it, it's a good system and probably becomes more and more useful the busier you are. I'm sure it could be used alongside the app you shared. Which looks nice btw, I've been looking for something self hosted to use for a small team. I'll check it out.