So, after playing Sims 4 with most of the EP & Game Packs (what's the difference? GP is a cheap half priced EP?) I decided that I want to learn how to mod either Sims 3 or Sims 4.
Pros for Sims 4:
Sims 4 Studio, Sims 4 Mod Constructor - these make it easy to make some mods.
Python 3 - fairly simple language, and Microsoft didn't mess up the GUI so it can only work making windows programs (programs, not "apps".)
Pros for Sims 3:
Unity no longer supports Python (gasp!) and the bottom line is that EA/Origin will pull the plug on Sims 3 one day and the only option left will be to make your own game in Unity and that mean learning C#.
Tools like IL spy make it easy to snoop around existing C# code. I don't know how to do that in python.
No "Joker" smiles on the sims.
I was, at one time, a competent C programmer (not coder, programmer!)

So, the skill set I see are:
1) Blender for meshing and animations (not Milkshape, because $$)
2) Gimp for graphics (not Photoshop, because $$)
3) C#
4) Jazz (no one is sure what it is, and there's limited info on how it works.)
5) Learning the Gordian Knot of EA spaghetti code used to make the sims.
6) S3PE, S3OC (Fairly competent with those already)

What am I missing from my Sims 3 modding curriculum?

Not much, as far as I can tell. Then again, I am but a lowly novice when it comes to most of it. I've gotten serious mileage out of both GIMP and Photoshop, and with a bit of effort I can find my way around Blender or S3PE as well. But I can't seem to get Blender working with TS3's file formats so it's out of the question for modding TS3, at least on my end. And all I know about C is that I'm best off staying the hell away from it.

So from the sound of it, I'd say that's a pretty complete skill set you have there but I could be wrong entirely.

it's not a skill set I have. Right now, I'm trying to learn Gimp. Like most tutorials, the software version (2.10) is way ahead of the tutorials version (2.8). Okay, had enough of that, went back to Gimp 2.8.
Then the tutorial author flippantly starts using the tool options dialog, but never says how to bring that up and the default install is set where tool options don't show up by default. You have to dig around and you find out you go "windows"->"dockable dialogs" (not "Dockable windows" as the help file says)->"Tool Options".
As someone who had do design stuff and then document it for techs and sometimes teach it, I find this very frustrating. But I understand the problem for the tutorial being written from someone who understands it and doesn't know what a new user doesn't know.

Can't help you on that; I learned GIMP entirely by myself, and on a now horribly outdated version. I have one of the, if not the newest version running on my laptop and I hate it, to be honest. I use 2.8 on my PC and it works just fine for me.....as far as GIMP works. Sure, the new one seems to have a bunch of fancy new features (because it's hardly ever been as advanced as Photoshop to begin with) but overall I'd prefer to stick with the older version and use Photoshop or Illustrator for any of the more in-depth stuff.