I just use the Hubspot form listener. This just pushes an event to the data layer that I would use to trigger GTM to capture the custom event.
<script type="text/javascript">
window.addEventListener("message", function(event) {
if(event.data.type === 'hsFormCallback' && event.data.eventName === 'onFormSubmitted') {
window.dataLayer.push({
'event': 'hubspot-form-success',
'hs-form-guid': event.data.id
});
}
});
</script>