I just run the official docker image with the relevant bit of enabling CORS
$ mkdir /tmp/geoserver
$ docker run --rm --name geoserver \
-v /tmp/geoserver:/opt/geoserver_data \
-p 8080:8080 --env CORS_ENABLED=true \
docker.osgeo.org/geoserver:2.23.x
Without any further configuration, I could add the nl:countries world layer to Kibana Maps just loading the WMS endpoint
http://localhost:8080/geoserver/ne/wms
Because the layer has a default style I don't need to select any and it just loads fine.

I would recommend starting from this initial setting to test things.