Righto!
point 3. Up the shutter = up the ISO. There's no defying the laws of physics(in this instance the physics of optics and electromagnetism!! )
after point 5 you should note that it may be imperative that you start the process of cataloging religiously!
It doesn't seem important, but if you end up like some of us(I won't point fingers at anyone in particular .. like someone who's name starts with an A .. and maybe a K in there somewhere too... ) you could end up with hundreds of thousands of images that you just cant find easily due to the sheer number of repetitive image types you have captured over the years. Cataloging now eliminates the need to do so in bulk at a later date when you finally realise that it's important for the sake of your sanity.
You don't know where the hobby will lead you, and cataloging could ensure that this won't be to a centre that deals with mental health issues 'cataloging procrastinators'
Wasn't it Malcom Frazer that said life wasn't meant to be easy!
What I think he was referring too way back in 1971 when he said this, was that digital photography was going to be the bane of obsessive compulsive types(that forget to catalog their images! ) .. that can't yet decide on which 'process' is best for them.
I think he had the foresight to understand that digital photography was going to be a thing, and that thing could drive some folks stark raving looney too.
OK, forget my reply above to point 3.
I think the other way. Shutter speed is more important as too low and you get blur, you can't REALLY sharpen out blur, even tho some software try too. Of the three exposure triangle variables you can learn to master, the only one will be shutter speed.
The other two(aperture and ISO) are a must.
That is, a slower shutter can be used in some instances, once your proficiency has matured, and you could handhold a 600mm lens at 1/100s using good technique and support. But the others are dependent on physics again.
One of the reasons I suggested to use Nikon's free CaptureNX-D. the colour control point editing method is far simpler and quicker.
As an alternative to the sometimes woeful CNX-D(but I persist with it) is DxO's PhotoLab software.
Also nice to use as it has control point editing too, but it's far more complete in it's tools than is CNX-D.
Photolab has a free trial thingy bit.
The only advantage in using Nikon's software is there are some useful tools to align nicely with the camera.
I reckon that using LR or Photolab(PL) you may not be seeing what you see in the software looks like what you see in the camera at the time of shooting.
This is normal, as the raw file rendering processes are different relative to each other.
So if you had both LR and PL, and you compared the captured images shown by each software relative to each other, each one will display a different images(and hence histogram).
With the Nikon software WYSIWYG both in camera and on the computer later on. It also has some other interesting features that are a bit hard to fully understand as well.
As for noise, use the noise reduction tools in any of your software, they're very good nowadays. With your comments re noise, I'm assuming that you're cropping quite a bit, so hence those comments.
IIRC, I think you also got yourself a tripod when you got the Sigma lens? Have you tried to use it for wildlife/birding yet?