Which OptinStack event should I use for a GTM trigger?

Use a GTM Custom Event trigger with the event name optinstack_analytics_accepted. Similar events are available for marketing, preferences, necessary, and GPC detection. optinstack_consent_updated fires whenever the consent state changes and is useful when your tag needs to evaluate the complete choice object.

The category-specific *_accepted events fire once per page load for a grant. If consent is later denied, the accepted marker is cleared so a later grant can fire again in that session.

Documentation: Consent Mode dataLayer events and Runtime events.