18 August 2021
The LineItem objects were added specially for user convenience. These objects store Lines values from parent objects as separate records. They allow you to view these lines in a tabular form, with Query, export them to CSV with Export, import them from QuickBooks to a cloud application or database, where these lines should be stored in a separate table, etc. See https://docs.skyvia.com/connectors/cloud-sources/quickbooks_connections.html
These objects store the same information as the Lines field in the corresponding main table - Invoice, Bill, etc.
Thus, you can remove foreign key constraints between these objects and use them in different packages. The package with incremental updates and main objects will be updated often. However, you can run package without incremental updates more rarely to refresh the data in these tables completely from time to time (e.g. once per week, every 3 days, etc.).
Unfortunately, at the moment, there is no other option to overcome this limitation.