The sigma can do wireless flash as well.
I use fill flash outdoor when it is harsh light, too bright one size and too dark on the shadow side, but mostly with a ND8 filter or CPL pair. That to bring the synch speed down to 1/180 (or better 1/125).
Getting in the shade is another option, also a foldable reflector is usefull for portraits (or white shirt, paper, card board whatever bounce a little light back).
In my case, I want large aperture for bokeh, but also balance the bright ambient with the subject.
Off-camera flash is the way to go, but controlling the sigma with build-in wireless, shutter speed and aperture may be a nightmare. Sigma user manual is a nightmare to read itself. I found out the radio triggers in manual flash/camera control is better suited.
Just ask, if my description is not clear enough ;-)
Here's a sample:
ND8 filter, off-camera flash at 1/4 power, sigma 530 super.
Without flash fill, the T shirt would be too bright, the face too dark on the shadow side.