Just recently I went on a trip to Europe and had some amazing views (that I captured the best I know how in manual) of city and landscape. Unfortunately, for about 4 days the skies were as overcast and bland as it can be.
I have used sky replacement with PS Elements, but that is tedious and time consuming work. I search the internet and came up with a couple of options. In no particular order:

1. Landscape pro
2. Luminar
3. PhotoShop Sky Replacement
4. Befunky
5. PicStart

My questions:
what do you recommend or use?
How intuitive is it? Easy?

Better late than never, I guess.

I purchased the Sigma 100-400mm lens a few weeks ago and was quite surprised how sharp it was at 400mm compared to a number of my other Nikon lenses at full reach, 300mm. I wanted more of a reach for birding than what my 18-300mm provided, price was a factor along with weight (I'm hiking usually 4-6 hours at a time).

I initially ran a resolution test on the Sigma lens before testing it in the field and I saw no reason to make any adjustments (front or back focus) using the optional USB dock that I purchased with the lens. It was dead on with my Nikon D7500.

The lens focuses very quickly with the D7500, somewhat slower using my D600.

The following photo was taken with my D7500 and Sigma 100-400mm.
Settings: 400mm, f8, 1/1250 sec., ISO 1250, cropped about 30% in LR