Personally, I only play Sims I create for the game. Some of them already have a background, as in my Cantrell family. I developed them to fully flesh out my O/C, see my siggie below. What I wasn't able to do, since my O/C's family were minor characters, was to give them more 'time' in the story. I made Erik Cantrell the second eldest child a Rock Star. Now, that we have GF (and I also use Road to Fame) he's a Rock Star, once again. By focusing on his career and his love-life (which wasn't any better in Sims4 than in Sims3 LOL) I have had a blast.
The other game save is that of my Ancestors, whose lives I already have information on. I love them to pieces and it's fun to play them.
If you don't already love a Sim you're trying to play, that can get boring pretty quickly. Pick a favorite, and invent their story. I shared some of the stuff going on in my game, and it ended up becoming a blog.
Shameless Plug: https://erikcantrellscheckeredlife.blogspot.com/2018/12/chapter-one-cautionary-tale.html
I've had a slight hiatus, unplanned, due to my being a silly nit. Things went from bad to worse when the StrangerVille patch killed my mod. I did learn there was an updated version, but once I downloaded it, turned out my game save was corrupted. Worse, still, was learning my Sim, Erik, was also corrupted. Suffice it to say, I had to restart him from scratch. He's just now back to where he was. I will blog a new chapter once I have a story to tell.