I would initially do the experimental city as an Eve Online styled MMO - but with multiple servers, each with a specific type of government and economic rule. After a couple years of letting each server's population fill up and learn their niche, create a 'hub' server, where players of the OG servers can start with a territory and compete. Doing it this way, it would make it easier to remove outside factors like geography, pre-existing wealth, unfamiliarity with UBI, and so forth.

The way I figure, it would be easier to track success and population metrics with a wholly artificial reality, then use the insights from that to inform how to structure a realworld UBI Island.