Okay, this is a lot to chew through, but I'll try my best.
1. It is possible to install your games to any drive you wish. I made a folder in my E drive to act as a point of installation for all Sims games (2, 3, and 4), although you could just guide the installer to your D Drive alone and it will create a "The Sims 3" folder, anyway. But I like to keep them organized in their own little home, so I gave them a folder where they can all live in harmony. :lol:
2. I'm not sure why @Cororon chooses to make new folders for each EP and SP, but I just want to point out that you don't need to do this. When you install the base game and all of the expansions/stuff packs/etc, you just choose the same installation point (your D Drive itself, or a folder you made in it for The Sims) and the installation will detect The Sims 3 folder's existence and all of the EPs and SPs will coexist in the same "The Sims 3" folder as the base game data. Example of my installation:
https://i.imgur.com/c4u5Mfh.jpg
(And, yes, my general "Sims" installation folder is called "theorphanage" :lol: )
3. In the installation process, it gives you the option of choosing the installation destination. It's by default set to your C Drive, but you can hit "browse" next to it and direct it to your D Drive. I'll try to find something to install to provide an example pic lol But it might take a sec. Edit: As promised, an example:
https://i.imgur.com/AheVgoG.jpg
As you can see, every program has that default installation location. You can hit "browse" and navigate to your D Drive and whatever folder you want it to be installed in. If you don't select a folder, it will just create a "The Sims 3" folder to install itself to.
4. Custom Content from the launcher is apart of your User files and not the game files. Follow the instructions here to change the location of your user folders to your D Drive:
https://www.dummies.com/computers/operating-systems/windows-10/how-to-change-the-location-of-user-folders-in-windows-10/ Any game installed anywhere on your computer will be able to find your user folders, so you don't have to worry about what location you keep them in. And you won't have to delete your current user folders for TS3.