I'm beginning to become OK with term limits. However, I think it needs to coincide with Presidential terms somehow. In my perfect world, a justice would leave the court every 4 years. So, a Presidential election is in 2024, so in 2025, that President gets to nominate one justice. It's early in his/her term and not in an election year. The only way one party can stack the court is if, for some reason, a second justice also leaves in that term. OR....one party maintains the White House for an extended period of time.
The problem with that is, that means a justice is on the bench for 36 years. So, maybe my idea needs to be modified so that a justice steps down every two years, so in 2025 and again in 2027, giving that president two nominations. (your 18 years)
Also with my idea, each justice would know exactly when their term is up. There would be much less game playing on when they are going to "retire". That is, unless the justice wants to retire early....which, I don't know how to fix.