I want to rollup the subtasks fields (Main fields or the custom fields) to the parent task so I can build the dashboards base on that rollup result

I'm not sure if this is already possible but I would like to be able to create a custom field for a parent task that is the sum of each subtask of the parent task. My idea is to track my account balance in the parent task and to include each transaction as a subtask. I realise there is a calculate column option to determine the sum of a column but this sum figure does not appear to be accessible for triggering automations. For example, I would like to trigger an alert to top up balance if the balance in the parent custom field falls below a certain level. Perhaps all this is already possible and I'm not thinking far enough outside the box.