Umbrellas are a good, common choice for off-camera speedlights. A speedlight is not too directional to use with umbrellas, and white umbrellas in particular spread the light well regardless of the flash’s original beam pattern.

In general, use the same kinds of modifiers you’d consider for monolights; the small source size of a speedlight head is not fundamentally different from many monolight flash tubes. What mainly determines the softness of the light is the size of the modifier relative to the subject, not the fact that the flash is a speedlight.

So you’re likely overthinking the need for extra diffusion layers. Start simple: a white shoot-through or reflective umbrella is an effective, efficient option for location lighting with speedlights. If you want practical guidance on setups and modifier choices, Strobist’s off-camera flash and Lighting 101 resources are widely recommended by speedlight users.