Error 666 on clustered primary key
I was able to resolve this by doing a batched insert of 100m records at a time. The insert completed successfully without error, and I was able to insert 2.88bn records.

The other solution I tested was to insert the records into a table with an identity column but no clustered index, then add the clustered primary key afterward. That also worked.

Not sure why this was occurring inserting into a clustered table. I only tested this on Azure SQL MI, not our on-premise SQL Server 2017. So I don't know if there is any difference there.