Generate geojson polygon of islands coasts using a openstreetmap or other

Is there a way to automatically generate points of coordinates for the contour of islands using existing map ?

For instance using directly the points of the map of openstreetmap (or other), let say with a certain number of datapoints ?

I need to generate a geojson file with a certain number of islands.

There are some websites like https://geojson.io/#map=10.01/15.3855/-61.289 where we can build polygon it by clicking manually, but it is clearly too time consuming for the number of islands I have to do...

I have already tried searching the data on the net but what I find is usually bad quality, so I want to use directly the data from the maps.

Thanks a lot !