I'd definitely view it over a shorter time period than a player's entire career. Yearly or a short period of years. Not many guys are #1s for their entire career, those guys go to the Hall of Fame. Certainly you could make that you definition, this is all subjective, but I think that's too narrow to be meaningful. Another example for why I wouldn't need to define it over a many years stretch is because you kind of know right away and you also know when a former great has lost it. What a guy did last year doesn't matter, only what they are doing now. Ja'marr Chase and Justin Jefferson sure look like #1s to me, and they haven't done it very long. Julio Jones and AJ Green aren't #1 any longer, but they sure were for most of a decade.
I like the idea of defining it as someone an opposing defense needs to gameplan to control. I think that applies to more receivers than you are seeming to indicate here. And of course that also depends heavily on the defense in question, a team with Jalen Ramsey or another shutdown corner doesn't need to do very complex gameplanning against a top receiver, you just hope your guy outplays theirs.