Modern Auth and Unattended Scripts in Exchange Online PowerShell V2

Hi.  I am trying to convert a c# unattended application from using basic auth to the new certificate based method and have tried using several suggested options, the latest being a process based on that indicated by JrouziesM 12-16-2020.  However I still have problems with the following:

var certId = "xxxxxx";
var appId = "yyyyy";
var org = "zzzzz";

var proxyType = "IEconfig";

InitialSessionState initSession = InitialSessionState.CreateDefault();

initSession.ImportPSModule(new[] { "ExchangeOnline" });

Runspace Runspace = RunspaceFactory.CreateRunspace(initSession);

Runspace.Open();

PowerShell powershell_2 = PowerShell.Create();

powershell_2.Runspace = Runspace;

PSCommand session = new PSCommand();

session.AddCommand($"Connect-ExchangeOnline -ConnectionUri {strRemoteComputerUri} -ProxyAccessType {proxyType} -CertificateThumbPrint {certId} -AppID {appId} -Organization {org}");

session.AddScript("get-mailbox -identity xyz");

Collection<PSObject> output = powershell_2.Invoke();

However, this just seems to time out or not proceed to an output being generated from the Invoke().  No errors are trapped when I include it in a try/catch.

Is there something that I am missing in this approach?