I'll add my vote to the suggestion that you start your new families in the same save with your old families. It does a lot to add to the lore.
I would also suggest trying out a challenge to see if that holds your interest. I really liked the Zombie Apocalypse Challenge.
I remember when I used to play Sims 2 I would start a new family the way you do, with a general premise and a loose story line in mind. The personality AI for that game had a lot more impact, so often the game would push back at my plans because a sim didn't want to do what I wanted them to do (because of personality, not glitches). I could decide to double down on my original plan, but I would have to work harder at it, or I could go with the flow of where my sims were taking me. We don't really have that in Sims 4. If you don't enjoy micromanaging the details of your story to force it to be interesting, it can end up feeling samey. At the same time, the lack of push back makes the environment very flexible for playing quirky challenges, which is what I enjoy most about Sims 4.
