Forecasting Year Over Year

Hello, I am looking for a specific forecasting method. For context, I am working for a company that offers sports programming and education across many different winter sports. One of our goals is to figure out the total number of participants year over year and from there figure out new vs returning customers to find our retention numbers. We have succeeded in this, as you can see in the graph below.

I want to take this to the next level by adding some "what if parameters." I want to be able to forecast until 2030 (or a year of the users choosing), a goal of how many athletes to have by that year, and a goal of overal retention for each year. What I'm hoping this will give us is an idea of how many new people we will have to target every year to both reach our retention goal and total number of participants. Ideally, I would like to see the graphs above along with the years ahead of our forecasting that will show number of retained and new participants, which will add up to the total number goal of the users choosing.

My current measures are below (please excuse weird naming conventions, I am working on two different reports right now that are very similar):