How to apply Cluster Source for Circle geom in openlayers?

Question

I'm trying to apply clustering to circle features. Having read the documentation it seems that for anything different then a point a geometryFunction needs to be defined: my attempt looks like this:

var source = new ol.source.Vector({wrapX: false});

var clusterSource = new ol.source.Cluster({
    geometryFunction: function(feature){
        let circlePoint = new ol.geom.Point(feature.getGeometry().getCenter());
        return circlePoint;
    },
    distance: 0,
    source: source,
});

var vector = new ol.layer.Vector({
  source: clusterSource,
  style: function(feature, resolution) {
        var styles = [style1];
        if (feature.getGeometry().getType() == 'Circle') {
            style2.setGeometry(new ol.geom.Point(feature.getGeometry().getCenter()));
            style2.getImage().setScale(feature.getGeometry().getRadius() / (180 * resolution));
            styles.push(style2);
        }
        return styles;
    },
    updateWhileAnimating: true,
    visible: true

});

and an example feature I add:

circle = new ol.geom.Circle(ol.proj.transform([_lon, _lat], 'EPSG:4326', 'EPSG:3857'), 1);
feature=new ol.Feature({
geometry: circle
});

I tried this with 5000 circles with different distance argument for the cluster, and it shows them at first load then they disappear when I change zoom level. Any idea what I'm doing wrong?

Edit: styles used:

var style1 = new ol.style.Style({
        fill: new ol.style.Fill({
            color: 'rgba(0, 255,0, 0.3)'
        }),
        stroke: new ol.style.Stroke({
            color: '#737373',
            width: 2
        })
    });
var style2 = new ol.style.Style({
        image: new ol.style.Icon({
            src: 'https://www.freeiconspng.com/uploads/photoshop-tree-top-view-png-3.png'
        })
    });

Answer 1

Maybe a style function such as this, you would need to multiple the size of the cluster by a value appropriate to you application to get a radius

  style: function(feature, resolution) {
        var geometry;
        var features = feature.get('features');
        if (features) {
             // this is cluster
             if (features.length == 1) {
                 // cluster of 1, use that feature
                 geometry = features[0].getGeometry();
             } else {
                 // more than 1, use a circle based on size of the cluster
                 geometry = new ol.geom.Circle(feature.getGeometry().getCoordinates(), features.length * ???);
             }
        } else {
             // some other feature, not a cluster
             geometry = feature.getGeometry();
        }
        style1.setGeometry(geometry):
        var styles = [style1];
        if (geometry.getType() == 'Circle') {
            style2.setGeometry(new ol.geom.Point(geometry.getCenter()));
            style2.getImage().setScale(geometry.getRadius() / (180 * resolution));
            styles.push(style2);
        }
        return styles;
    },