Probably a better question for the wine forum, but I’ll take a crack at it.
I’m a bit suspect of the addition of sugar to the wine. I know why they do it. Its cheap, and it ferments very easily. But, if its a quality product your looking for I would definitely start adding more hawthorne berries, or if you don’t want too much of that flavor, you can add many many other sugars (honey comes to mind, would make a very interesting melomel).
Other than that, this seems to be little more than any of the “Wine from grape juice” style recipes that you can find all over the web. They’ll work. They’ll be drinkable, and cheap. If thats what your after, by all means go for it. If your after something you can truly share and be proud to say that you made, then I would suggest investing a bit more into it.