Bulk change of privacy settings for past activities

As you got it working, it's good.

Anyway it's also possible to use similar script in web-browser console (F12 key), so here is my script, tested in Firefox. Edit the search link to filter activities you want to change - and change typeKey to privacy level you need.

Below it's limit=1 for testing if it works as wanted, it will change only first latest activity to private.

It's normal search link from Connect Web, so it accepts all filters visible on the website, if you need more filters just check them on the website and add them by &.

jQuery.getJSON(
    'https://connect.garmin.com/modern/proxy/activitylist-service/activities/search/activities?limit=1',
    function(act_list)
    {
        act_list.forEach(
        function(act)
            {
                fetch('https://connect.garmin.com/modern/proxy/activity-service/activity/' + act['activityId'],
                {
                    'headers': { 'Content-Type': 'application/json', 'NK': 'NT', 'X-HTTP-Method-Override': 'PUT' },

                    'body': '{"accessControlRuleDTO":{"typeKey":"private"},"activityId":' + act['activityId'] + '}',

                    'method': 'POST'
                });
                console.dir(act['activityId'], act['activityName'], act['startTimeLocal']);
            }
        );
    }
);

Parameters:

typeKey: private, subscribers, groups, public
?limit= number of activities to change, starting from latest
&start= start from nth+1 activity, counting from latest
&activityType= running, cycling, fitness_equipment, walking, other
&startDate= YEAR-MM-DD
&endDate= YEAR-MM-DD
&minDistance= in meters
&maxDistance= in meters

Example:

.../search/activities?limit=200&start=200&activityType=running