Any way to "close" a group of reminders all at once on OSX?

There are two fairly easy ways to do this:

Option 1 (default):

With the default configuration of your mac, you can simply option + click the notification center icon in the top right corner of your screen. This will enable Do Not Disturb, which automatically dismisses all notifications and silences incoming notifications. If you wish to continue receiving notifications, you can option + click the notification center icon again. Your notifications that were on the screen will not re-appear.

Option 2 (creating a keyboard shortcut):

1. Open System Preferences > Keyboard > Shortcuts > Mission Control
2. Enable the checkbox next to Turn Do Not Disturb On/Off
3. Click none to the right of the option
4. Set a shortcut.

^{Credit: igeeksblog.com}

Whenever you want to dismiss all notifications, press this shortcut twice.