In the past I've used batches of data, inserted into a separate table with all the constraints turned off and using UNNEST, and then inserted into the final table once it was done. We ended up both batching the data and using UNNEST because it was faster but it still let us resume midway through.

We probably should have been partitioning the data instead of inserting it twice, but I never got around to fixing that.

COPY is likely a better option if you have access to the host, or provider-specific extensions like aws_s3 if you have those. I'm sure a data engineer would be able to suggest a better ETL architecture than "shove everything into postgres", too.