@nelluk - thanks and welcome to MobileRead.
I'm afraid I don't quite understand what you are hoping to do, and why you are using both of Download tags from shelves and Sync functionality? Can you give some more details and examples of your complete workflow - imagine you bought a new book tomorrow, where/what would you do with it outside of the obvious of adding it to calibre?
My own usage of Goodreads/workflow I would suggest is very different to yours
Strictly speaking I would argue that the Download tags function is really just a "less powerful" version of Sync. Download tags has the "advantage" of allowing you to do it for just a subset of your books, but in every other way "Sync" is the more flexible and featured option. However as I mentioned in my last post I don't use "Download tags" at all, it wasn't in my original plugin design and was only added in response to requests on this thread. So I don't know enough about how people use it.