How to create a legacy and stick with it

That's fair. If being more hands-on about their traits and aspirations would help keep you invested I'd say go for it. I myself randomise all traits and childhood aspirations, but by the time my sims enter their teens I usually pick their aspiration based on the traits they've received or what I think would fit them, or simply by what I want to play through. I will occasionally randomise it if I don't have preferences at the time, but ending up with, say, the vampire family aspiration when I really don't feel like messing with vampires it will just end up being a drag and I'd much rather do that aspiration with a sim that fits at a time when I do want to explore vampires again.

Really just mess around with the playstyle and rules and see what works best for you.