How do you stay invested in your save?

I have 656 hours in the sims 4 alone, and my dark secret....I'v never played past my original sim. I just can't seem to stay invested after hitting a new mile stone (buying a house or having a child). I'v had so many saves, and try to stay invested by making myself do everything vanilla with no cheats, but as soon as Iv grind and get a $150,000 house vanilla or max out my sims career I just dont know what to do next.

I'v tried challenges or setting goals for myself but I always end up creating a new save. What do yall do to have years old saves with like 10 generations??? I remember when I played sims 1 -3 as a kid I could play 1 family for multiple generations but I cannot seem to do it in sims 4.