If you're using the DJI camera on your Plus and saving your images as RAW (new firmware option), you can use the new DJI lens profiles in Adobe Lightroom or Photoshop to automatically take the fisheye distortion out.
I bet there's a lens profile for the GoPro, too, by now... I just don't know for sure since that's not what I use.
But, yeah, until you take out the distortion, there ain't no way it's ever going to stitch together right.