That's really pretty small if it's THIS one.
3.5 inches square isn't going to soften much at normal distances.
Remember: Bigger is softer.
A cheapo umbrella would be more useful if you want to use this with your new 85mm for portraits.

You could even bounce the light off of a large piece of foamcore.

The smallest softbox I have is 24" and I use it close to keep it soft.
The largest I have is maybe 5 x 7 feet.
I rarely use it, but when I do, it's with my more powerful studio lights.

When I started working in a real commercial studio, after 15 years working at a newspaper and university, I realized how much I didn't know.
You don't know what you don't know.

Add an umbrella and stand and flash adapter to that order if you don't already have one.
Personally, I like softboxes better. Not as much spill.
Have a couple of these: https://www.adorama.com/glsb3131sbk.html I use with speedlights for on-location work.
Use the normally useless diffuser cap you may get with the speedlight to even out the light inside the softbox.
This system sets up really fast- had 15 minutes in a busy restaurant to shoot a chef for a charity auction.

BTS (Behind The Scenes) and final shot below.

I think you'd be better off getting the Yongnou Transmitter for off-camera use.
Don't believe the Yongnou flash is fully compatible with Nikon's in-camera infrared commander system so controlling the slave remotely won't be possible, and TTL wont work (though I rarely use TTL with a remote) unlike the Nikon SB-700....but the SB-700 has to be in sight of the camera's commander unit to work reliably. You could just put it on a radio receiver.

Have fun with off- camera flash!
Opens up a lot of possibilities.
Joe McNally's books and website are a great resource as are Neil van Nierk, Strobist and several Facebook sites like Learn to Light OCF site and the Facebook site: Strobist .