Personally the flash is the last thing that I let dictate to me.
Aperture is typically my first choice, to get DOF right for the subject or style of shot.
Time limits are next in line, subject matter again dictates here but also sync speed. Typically for me it relates to age and activity, 1/30th may work for adults around a campfire with camera support, for youngun's or moving subjects or hand held I want to be up close to sync speed.
Aperture and time and background lighting (that I can't change) then dictates my EI.
Finally after all that is addressed then I start setting up the strobe for the subject. The reason I do strobes last is that they are adjustable and all the other choices are taken in my world.
With practice it can be easy and fast because we all find norms that work for us. Personally if I'm outside and there's a strobe attached to my camera the aperture is f/2.8, the shutter is between 60 and 125, the camera is on a monopod, the EI is judged by the situation, and my Nikon speedlites are on "A" or matrix TTL with no compensation.