By 3rd dan one could learn all of the strikes and blocks perhaps, but not master them. That is inherent in considering 4th dan a full instructor rank.

I would add that in some palces or schools, maybe this can happen. I would also guess that it can also take place sooner, depending on the depth of the syllabus & quality of the instructor or instruction.
However my point, which may not have been fully grasped is that the TKD that I still study & am still learning, which I do my best to share, is more an Art of SD whose main thrust is to build better lives. Given that, I constantly learn better ways to defend, including discovering that when trained realistically previous held assumptions become clear that they don't work. When it comes to the DO, the longer I keep at it, the more that is revealed to me or should I say, the more that becomes clear or apparent to me.

So IMHO TKD is a life long journey of learning. It would be my desire that more experienced that which has helped me grow so much.