HELP! How do you take an existing image of a circle and thin the brushstroke?

If the stroke is outlined already, then you could try to use the offset path function and plug in a negative value to make it thinner. Another easy option is to add a white stroke to it.

I don't understand English well, so I'm sorry if you get a strange answer.

Do you want the image brush art to be a thin brush art?

My method is as follows.

Open the image in Photoshop.

Move to the center.

Rotate so that the joint is centered.

Increase canvas size.

Convert polar coordinates with a filter.

Make the path.

Bring it to Illustrator and make it a brush.
Adjust the line thickness.

If I am saying something strange, ignore it.

So the brush is already outlined? And in which way should it be thinner?

You could try out the existing brushes, maybe it's one of them.