A large chunk of PdfPig started as a port of PdfBox (Java) , so it might be worth considering containerising or in some other way wrapping the PdfBox functionality to get Apache 2 licensed PDF merging [0] if this works for your scenario.

It's useful to know that this is a real use case too, I always assumed it was implemented 'just because' but the scenario you describe makes sense.

[0]: https://pdfbox.apache.org/docs/2.0.1/javadocs/org/apache/pdf...

Edit: another option is this pdfium wrapper for NET Core though it merges one pair at a time: https://github.com/GowenGit/docnet