I'm flying on a Mavic 2 and trying to figure out why my photos are grainy.  I've checked the ISO settings and what not (see settings below) and I'm not seeing the issue.  The setting is well lit with settings that should work for simple shots.  Thoughts are welcome, hopefully I'm just doing something silly.
Shot in Raw D-log
Size: 39MB
Dimensions: 960x640
F-stop: f/4.5
Exposure time: 1/2500 sec
ISO: 400
Aperture: 2.9

Google link to shared image:

https://drive.google.com/open?id=1gOAbX2CueGHP4gOwunZWKDm9zh4zWi0s

First up your dimensions are wrong. A raw image should be 5464x3640 zoomed in that far will show some pixelation.  
Second there is no reason to be shooting with the settings you are using.  Lower your iso to 100 and reduce your shutter speed.  you should try to use full f-stops.  Fulls are 2.8, 4, 5.6, 8 and 11.  Half stops or third stops should only be used if you can’t get the correct exposure setting adjusting iso and shutter speeds.  it’s not that grainy for a zoomed in and cropped shot.  

Third it’s shot in raw which meanings that you still need to develop the image with Lightroom or some other raw editing application.  If you don't want to develope the image shoot in jpeg.

Ok so the dimension in your post are wrong.   It is the correct size 5464x3640.  I downloaded it and threw on a quick edit. Reducing the iso will remove some of the noise however a 15% noise reduction in Lightroom smoothed out the image.  It looks fine.  

If you are zooming in real far when looking at the picture you will see noise.  For example if I zoom into the people on my 10in iPad I just start to see noise.  But that is somewhere around a 5x zoom.  If you made a 12x18 print you won’t notice it.  At 5x zoom that would be a 60x80in print with you face 10 inches from the picture.  You will see pixels that close.  A 5464x3640 printed at a fairly high dpi like 300 can be printed with no visible pixelation @ 12x18in.  If you were to print it to put it on a wall 24x36 would be fine with that photo as long as you stand at an appropriate distance say 3-5 ft away.  

DNG will usually be more noisy since DJI did not apply any reduction to them.
You have to take care of this yourself in post.
But you can do the obvious things.
- ISO 100
- Expose for the right.