If a monthly journal entry requires data from a system not yet integrated (like Square), what's the workflow today for getting that data into Stack?

For a monthly export not yet natively integrated, options today are: bring the file to your desktop and, when the checklist task/agent runs each month and flags that it's waiting on that file, attach it manually; or connect Google Drive so Stack has visibility into a designated 'stack folder' and can query files placed there directly without needing to be given them each time. File management integrations are expanding soon to SharePoint, OneDrive, and later Box and other providers.