Hey spirit, I would just like to caution you about the sigma lenses. They are known to have CA ( Chromatic aberration ) issues. Even when getting up in $1000* lenses, they still come around with that issue. Tameron used to be the same way, but in their newer lenses they really stepped up the game. So personally, If you cannot afford a Nikon (you can great used deals) lens than Tameron often outperforms sigma depending on the field. Also I would just like to know, Nikon I uses 100% glass in their lenses, however Sigma and Tameron have used some plastics in their lenses. Canon has also, but not as much. So these are just some notes to think about with your sigma, and as a proof aswell I Outlined and cropped where you can see the CA issue at large.
There are more spots, but that is where it is pronounced.