What is the proper way to display the full InnerException?

Just use exception.ToString()

https://docs.microsoft.com/en-us/dotnet/api/system.exception.tostring#remarks

The default implementation of ToString obtains the name of the class that threw the current exception, the message, the result of calling ToString on the inner exception, and the result of calling Environment.StackTrace. If any of these members is null, its value is not included in the returned string.

If there is no error message or if it is an empty string (""), then no error message is returned. The name of the inner exception and the stack trace are returned only if they are not null.

exception.ToString() will also call .ToString() on that exception's inner exception, and so on...

You can simply print exception.ToString() -- that will also include the full text for all the nested InnerExceptions.

I usually do like this to remove most of the noise:

void LogException(Exception error) {
    Exception realerror = error;
    while (realerror.InnerException != null)
        realerror = realerror.InnerException;

    Console.WriteLine(realerror.ToString())
}    

Edit: I forgot about this answer and is surprised no one pointed out that you can just do

void LogException(Exception error) {
    Console.WriteLine(error.GetBaseException().ToString())
}