For all it’s worth I don’t use sigma lenses anymore.
Before you go off, hear me out. I’m a basically broke fresh 18 with an interest in photography, I started when I was 14 and by the time I was 16 I bought my own DSLR, a Nikon D7100, I didn’t have any lenses so one of Dad’s friends basically gave away his sigma 50-500 f4-6.3. I tested it and it all seemed fine, I also bought a sigma 28-200 f3.5-5.6
After about two months of usage (both of them were second hand obviously) I started noticing loss of sharpness and very slow autofocus. After losing a job due to one of the lenses I basically put them in storage and left them. To this day I take out my 35mm and use it instead of those two. They’re basically heaps of trash