How do you keep it interesting?

I play rotationally so when this happens I can switch to another household. It's also really interesting for me to see how all the relationships from my different households connect.

Yep, swtching households and setting long-term goals for Sims and households to slowly work towards.
But it really depends on what you like to do, whether you build or not, whether you have aging on or off etc.

Definitely agree on playing on rotation. By the time I start to get bored, it's time to move on and work on the goals of a different family.
I also try and encourage myself to incorporate things in to my game play that I otherwise wouldn't have, like certain aspirations, trying to fulfil whims to see where that takes me.
Writing the story of my legacy helps too as as soon as one chapter is done I want to then start the next one and see what else happens in my characters' lives

I just play something else for a while.

I rotationally play saves rather than within a game. That way I slowly finish the challenges I start (a lot of them can be quite a grind so you need a break). I also have happy family saves and everything goes wrong drama saves and play what I need to suit my mood on the day. I never erase saves because I never know when inspiration will strike for those particular sims.

I like a story, sure the reality is if you dedicate some time to a reasonably sized family you should see everything but for those of us that find the thing we prefer that doesn't matter. Maybe for you, you need a story from a let's play to stick to copying doesn't mean anything everyone can have a family, start up some drama or stay chill..stick to building or creating sims or content whatever, that's just how the game is. Maybe try a story that appeals to you but keep everything on short if you have seasons pick the shortest amount and turn aging on short so you're only with sims a limited time and almost always preparing to move on. Then of course maybe that's also just what you like starting over getting in some play time then moving on completely that's fine too.

When I start to get bored, I look up other challenges I can do to keep myself entertained and also explore other aspects of the game that I haven't explored much or at all.

...and if there is a sale going on or I'm doing okay financially, I will treat myself to a new pack! Okay...that's kind of cheating because I'm basically buying myself out of boredom, but it works!

I play rotational and also I have a few occult families in my rotation just to change things up every now and then.

MODS! Lots of mods~

I’m a rotational player as well, play with aging off, and slowly develop my sims’ stories. I never spend more than a sim week playing one household before I move to the next because I get bored playing one family for an extended period of time. Also, I’m always coming up with ideas for new sims/families, so there are constantly new sims added to my game.

For instance, I just expanded (again) one of the first families that I created. This time I made all of the parents of the married couples because I wanted my family trees to show how all the children, cousins, etc., were related to each other and I wanted the kids to have grandparents. The back story for one set of parents is that they moved to Sulani after their daughters went to college. In Sulani, they became conservationists and had a twin boy and girl. I’m starting their story with the parents as elders and the twins as homeschooled teens. The twins will live a carefree island life, which will differ greatly from the structured life their older sisters had as members of high society and debutantes of Willow Creek. I also created Sulani families with teens so the twins will have their circle of friends.