Is there an API call/ clicky button in the UI I can use to force a 1-off sync?
You can run a warehouse sync from with the InsightVM UI as follows:
Go to Administration → run
In the dialog box, type export warehouse
You could do the above steps through the API by running a post against endpoint /api/3/administration/commands
Place the command you wish to run within the body of the post, in this case export warehouse