Target share is not Amari's problem and his end of season target numbers are very deceiving. Through 10 games Cooper was averaging 8.4 target/game which would have been in the top 15 among receivers. He was incredibly inefficient.If Oakland commits to using him on a more consistent basis and giving him a proper target share (he was targeted as often as Mo Sanu and Marqise Lee this year) then I think he could flirt with low-mid WR1 numbers with a high WR2 floor. The fact that he can be such a solid deep threat and, as you said, the most efficient slot WR over the past two years, just proves to me that he has legit top-end talent and can be a true #1 in the league.
Cooper is young and could still become an elite wide receiver, but as of now he is the most overrated player in the dynasty community.