I don't think programmers are the issue here. What you describe sounds to me more like the typical product management in a company. Stuff features into the thing until it bursts of bugs and is barely maintainable.

I would love to do something like what you describe. Build a simple but solid and very specialized solution. However I am not sure there is demand or if I have the right ideas for what to do.

You mention invoicing and I think: there must be hundreds of apps for what you describe but maybe I am wrong. What is the one good app you mention? I am curious now :)