If you want soft light and minimal shadows on portrait, perhaps on camera flash isn't the best way to go.
soft shadows has a lot to do with size of the light source rather than the diffusion of the light itself. This is one of the reasons why small and portable umbrellas are often used with speed lights. Those on-camera flash diffusion attachment doesn't really change the size of the light itself. It just scatters it in all directions and basically weakens it. If you do that under daylight conditions, you will not have enough power to do much of anything except as a weak fill. You won't be able to over-power the sun. Also, frontal lit portraits aren't always flattering.
If I were in your position, I will first try to find locations with open shadows. I will also try to use large reflectors and when necessary, overhead diffusers. Only then, I might use OFF camera remote flash for catch light.
In my opinion, the location you are describing isn't the best for what you want. If you can't change the location, then so be it, but if you can, I'd do that first.