Since FluentApi is a source generator, you have rightfully mentioned in the readme, that PrivateAssets="all" should be configured. This causes issues when using the generated APIs if FluentApi is not referenced in the project which depends on the project using the source generator:

System.TypeLoadException: Could not load type 'IGeneratedApi' from assembly 'ProjectUsingFluentApi, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'

This is because all usings in the source file where [FluentApi] is used are copied over into the generated code, including usings of the M31.FluentApi.Attributes namespace.