A new option is being deployed for Office Click to Run installation for Outlook.
This option is going to set the Reminder pop up on top of all other windows.
To get it,you need to use Office Click to Run and have the version/build 1805 / 9307.2004 (or later).
And in case you are not aware, you may have to restart Windows (not only Outlook) 3 times before the updated build kicks in.