One idea for the Rookies could be to have only Rookie Leagues, something like Rookie World Series 1, 2, 3, etc. or Great Britain, USA, Singapore, etc... with all the existing tracks. To move up to Pro, the team needs to be level 10 and finish in the top 3, 5, or 7 (not sure about the exact number). One league per hour or more leagues depending on demand. You should have data to know which hours are in highest demand.
Rookie Leagues would always start and end on the same day, and the remaining teams would be randomly rearranged so they don’t always compete against the same ones.
To maintain interest in the Rookie Leagues, there could be rewards for competitors at the end of the season, such as tokens, level 10 drivers or CD, and so on (for those who qualify for Pro). For those who stayed to compete in another season of Rookie, there could be monetary rewards ($$$).