Adding InnerException helped track down the problem assemblies. Some were missing, but the final problem is the nuget build for iText7 seems to have an incorrect reference?

Could not load file or assembly 'System.Text.Encoding.CodePages, Version=4.0.2.0, ...

Looking at nuget, there is no such version. It appears there's some dll in iText7 that is referencing it (even though the iText7 project files don't as far as I can see).

I substituted the iText7 dlls found for iText7Module on PowerShell gallery and targeted 7.2.0 (which is what they are labeled) and it now works. The iText7 dlls from iText7Module labeled as 7.2.0 aren't the same as the ones with the same version from nuget. Not sure what is going on exactly, but it resolved the issue.