I ran into a similar problem recently. Basically, we have a governance system to record any team created. And I needed to know whether the provision was ready or not. My solution was to have a "Governance" List, where each item is a site created. the list also has a column "isReady" that I update to "yes" when my powershell script is done provisioning. So that's an option for you. Basically, you have a list to keep tracking of provisioning, have your script set(or create) an item in this list, and create a Flow to check, when an item is updated(or created) if it is ready, aka finished provisioning.