I love it that you called it “RockCare”, to rockfinity and beyond... 🙃
In all seriousness, IMHO I don’t think this is something worth developing for others to buy (nor as a free module). Everyone has their own set of requirements (and their own way of doing) and this would make it a very hard time on you to figure out a “one-size-fits-all” solution. I would advise to save yourself this burden, knowing you’re already going on big endeavours with other modules (e.g. RockCommerce).
However this is just me and you could definitely beta-test it with select users to see if they could adapt to your way of doing and in this case foresee a potential public release.
In my particular case though, this is something I wanted to tackle for a while now, also with ProcessWire. But I know for sure that I would want to do it all by myself, as a way to experiment (and have fun like you said) but also to make sure it’s perfectly tailored for my usage.
What would be nice though is having a thread (this one?) so people can share what they put in place (PW-based that is) to help them managing this aspect of their work. Not going as far as sharing a whole site-invoice profile, but something along the lines of the associated blog post 🙂