Hey, thank you for the suggestion. I will give it a try. :)
I see the point, that it is more clean to separate the counter and the theorem functionality, and it is absolutely no problem for me, but I think many end users, who don’t think too deeply about which solution is the best, would prefer to just have to call one package to get working theorems with custom numbering out of the box.
So I suggest you to maybe call rich-counters inside great-theorems automatically?
Edit: @jbirnick I actually asked a question about your package here.