I'm wondering how many times you've redone your islands - either flattened completely and redid everything again, or just moved everything around.
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I have two islands.
My main island is my keeper: What I have acquired can be used for a second island. That second island is not intended as permanent.
• Island #01: Began in March 2020. No reset. In June 2020, I finally realized how to design it. Terraforming, much of it in between March to June, ended after this point.
• Island #02: Had one competed [enough] island which has since been reset. Replaced with another island. Now, that replacement has a current island (again, not expected to be permanent).
I don’t make a point of keeping track of how much terraforming I have done (first island) or will do (secondary island). I think that is for the best.
I have experienced all four airport colors. Primary island has a yellow airport. So, things have worked out for me.