The Missouri plan isn’t that wonderful. Like other Progressive Era reforms, it takes power away from voters, who are assumed to be incompetent to elect a judge, but then lets the voters decide whether or not to retain a judge — based on what criteria?
If we’re going to amend the Constitution, the Federal Reserve Board is a good model: Staggered 14 year terms, so that almost every President gets a chance to pick 1–2 justices. Few justices die in office or unexpectedly retire. The justice must be confirmed by the Senate, but there could be a mandatory time limit on confirmation, so that if the Senate doesn’t act within the time limit, the justice is considered to be confirmed.
Just convince 2/3 of the Congress and 3/4 of the states and we’re good.