Create CB Auto Action to send AcyMailing on CBSubs Pending Invoice

We have CBSubs plan with option of offline/check payment.

We would like to create an Auto Action that will add a user to an AcyMailing list if their Invoice is still pending 30 days after the date it was created.

Ideally we would have this triggered via a cron job that runs overnight. It can NOT depend on the user who placed the order logging in.

Can you tell me what the Global Trigger should be and what the Conditions should be to have this action happen?

You should be able to just use CBSubs Mailer for that then setup CRON for your mailer automation URLs. Aside from that there's no trigger for this behavior for CB Auto Actions to act on. CB doesn't just fire triggers on a schedule or anything like that. Triggers require someone to trigger them.