I too played the Sims for years before settling into a play style that really suited me. I’m a simple family player — challenges and trying to meet goals doesn’t really interest me — but like you, I used to get bored of playing with a family after a while. Then I discovered rotational play, and I’ve been doing that consistently for about 4-5 years now.
There are different ways of doing rotations. My version tries to explore all the packs (I’ve got all EP, GP and SP so far) so there’s lots of variety. I’ve got one save game per world (so I’ve got quite a lot of saves now!), and in that save game I play one household that focuses on the particular elements of that world/the pack it came with. So in my Evergreen Harbor save I play a household that focuses on eco lifestyle stuff, in Del Sol Valley I play a rich and famous sim household, etc. I only play each household for 2 sim weeks before moving on to the next save game/household. Ageing is set to Long, so I have plenty of time to do what I want — but I tend to age sims up when I feel ready to move on, rather than waiting for them to age up automatically. I find that I’m getting really attached to the households I play, so that when I return to a particular save and household after time away from them, I’m excited to play them again! This way stops me getting bored with a family, helps me to explore packs as fully as possible, and also means I don’t forget or neglect any of the worlds.
I hope you can find something that works for you!