Can you install the game on one drive and expansion packs on another

Simply put; No.

For occult status, only the parents are checked.

Of course, if the human child were to later become a vampire, their children would be able to inherit it. ;)

This is probably not going to fix the entire issue since I've never encountered save times as long as you are describing, but I've noticed that the save time also depends on what is currently displayed on your screen.

What I mean is that performing a maximum zoom in the middle of a room or in the garden seemed to reduce the save time. In my case it was maybe going down from 1 minute to 30 seconds, so I'm not sure about the impact it would have on your side, but it's still worth a try.