There was a gap of one year between the events of John 6 and the institution of the memorial.
DW: 1) it is called "progressive revelation" and he also said, there are many things I should tell you, but you are not ready yes" or words to that effect
2. If Jesus included everyone in "anyone who partakes", then why did he institute the memorial with only his 11 apostles? Is there an indication that he invited everyone, and only his 11 apostles turned up?
answer: The passover was a family celebration, and that was his closest spiritual family.
just commenting, not endorsing the talking snake story's veracity.