I do not really play my sims, I play their world. In the beginning I think of a setting and what houserules I want (are all jobs available to everyone? how about rocket ships in the backyard? is magic real? how much does a servo cost? that sort of thing). Then, when I create my households, I only need to decide how they will fit in.
A simple concept like "ex career soldier" is good enough, because my backstory already suggests all the other details, like where that sim will have been stationed. I just have to pick from "Strangerville Counterspionage", "Detroit Android Revolution" and "Sulani Coast Guard" and each choice will come with a handful of existing sims that my new one will most likely already know, whether as comrades or as former enemies.
Or when I want to play an archaeologist, a government sanctioned selvadoradan historian and a young adult from Detroit desperate to pay the bill for the mansion they inherited will actually play different: the latter will get put into the next plane home if he runs into line of sight of a local in the jungle, while the former can expect the occasional cheat for equipment. So basically I think less of who my sims are and more of what role they play.
Getting a save off the ground from scratch can take weeks before I even load up the game for the first time. But in the long run it is worth it, because no sim will ever exist in a vacuum and think "What should I do now".
The downside is that I often has to declare phone calls or chance cards as "never happened", because they contradict my backstory.