Fusion exports models very messily, so I brought it into Maya for retopology. I used Quad Draw to re-make the model, this time with cleaner UVs and less polygons. I unwrapped the model, making sure to straighten any UVs I could.
Thanks to the retopology I did, the model was now much easier to texture and took up less space. However, it was also much more jagged and rough. This would be fixed in Substance Painter.