Noise on EOS R8 Image

[HB502]

I recently bought an R8 and have been trying my hand at photographing some sunsets.  However, it seems that whatever I do, I get a lot of noise in the image.  I've tried wide and small aperture (f/4.5 - f/11), auto ISO and setting ISO to 100 to force a shutter speed, tripod and not, and changing the exposure / metering settings.  Example attached (this one is 24mm, f/4.5, 1/1600 shutter, ISO 200).  The clouds / sky in the picture (and in all of them) has a grainy texture (noticeable on the full shot at normal zoom, and much more so when magnified). 

I'm stumped on what might be causing it and how I can get this resolved.  Any help would be appreciated.   

 

[HB502]

Very nice results with the jpg.  I'm new-ish to all this.  I did a little post processing on the phot and was kinda happy with what I'd done... until I saw this.

For what it's worth, I've been using a depth of field calculator in an attempt to ensure everything is in focus.  The shot was either or 24mm or 50mm and [supposedly] 24mm f/4.5 has anything beyond my height in focus and 50mm should be beyond 10yd (hyperfocal near limit).  I don't have too much practice though, so that may not be practical.  

I've been trying to get as much light into the sensor, but it sounds (and looks) like I can get the biggest bang for my buck by improving my post processing skills first. 

Thanks for your patience with me, and if there is any books on lightroom (or an alternative post processing solution you recommed), I'd be interested in hearing.