Hi
@metallpea !
I've completed Pinstar's Legacy Challenge twice with The Sims 4.
For TS4, there is a rule that each member of the active household can extend their life with a Potion of Youth once in their lifetime.
So, I think your current legacy challenge is not in violation of any rules. That's my personal opinion, anyway.
When I played my legacies, I usually had my founder, founder, heirs, and their spouses all drink a Potion of Youth on their Young Adult birthday.
With the passing years, The Sims 4 included so many careers that each of my generations had to complete 3 or 4 careers.
Anyway, I believe that your legacy is still "legit," imho.