Running Code during Dialog initialization, after GUI is drawn

I'm writing an application that communicates and initializes instruments (via COM ports) during the initialization phase of a Dialog (using MFC, on Windows 10).

The instrument initialization is presently taking between 4 seconds and 10 seconds. So the GUI won't appear until the initialization is finished.

I want to know how to perform the drawing of the GUI, then call the instrument initialization function.

Here's a code fragment:

BOOL MyDialog::OnInitDialog()
{
  //...
  // code to initialize dialog widgets
  //...  

  Initialize_Intruments();
  return false;
}

From the Microsoft Documentation of the CDialog::OnInitDialog method:

Remarks

Windows sends the WM_INITDIALOG message to the dialog box during the Create, CreateIndirect, or DoModal calls, which occur immediately before the dialog box is displayed. {emphasis mine}

I want to perform functionality, after the dialog box is displayed, during initialization of the dialog.
How to do this using C++, MFC, Visual Studio (2017) and Windows 10?

Notes:

1. The application is a single dialog that runs test procedures.
2. A splash screen is another technique under consideration.