Manipulating the map in JS - [GM] Unable to find map
At a glance, it sounds like it's just a matter of waiting for everything to finish loading. You are trying to call a map which has not yet been initialized.
Good news! The tag method includes a callback option which allows you to specify a JavaScript function to be called after the map has finished loading...
{{ map.tag({
    'callback': 'moreJavascript'
}) }}

{% js %}
    function moreJavascript() {
        var map = googleMaps.getMap('mapAlpha');
        console.log(map);
    }
{% endjs %}

Take a look at this practical example of how someone could use the callback option to enable marker clustering... I know that you aren't trying to cluster your markers, but the broader execution is the same.