How do I blend an island outline without overlapping?

Try a few itterations of offset path to get the first few outlines around the island to smooth out and eliminate the harbours, bays, and lagoons. Then use the outermost path to blend to a circle.

You’ll have to experiment to find the right balance between offset path (which is always uniform) and blend (which is not). In the example below I used two offset paths then blended from the last path to the circle.

That example probably has been made by Object > Path > Offset path a couple of times.

There are plugins that can create that more easily. But they are not free.