virtualbox.org

I pre-loaded the downloaded Linux .iso to the optical drive before starting the virtual machine so that I could simply choose it when running or installing Linux...

Now, Linux after installation and reboot is telling me to remove the installation media, but the option is grayed out in VirtualBox even though it appears to be loaded into the optical drive...

If that is grayed out, there is probably nothing in the drive. What does Mint say? If there is an optical disk mounted, you should see it in File Manager. (The screenshot has the GAs mounted.) In any case, you can add or remove optical files from Settings when the machines is shut down.

The virtual optical disk has probably already been ejected. Just press the Enter key.