Is it possible to manually sync reminders with iCloud whenever you want?
Yes, but only by revealing the Debug menu in Reminders. To do this:


Launch the Terminal app (usually found in the Applications > Utilities folder)
Enter the following command (or just copy and paste it):

defaults write com.apple.reminders RemindersDebugMenu -boolean true

Press Return
Now launch your Reminders app (or, if it was already open, quit it and launch it again)


Now you can go to Debug > Refresh (or just press command + R) to refresh your Reminders with iCloud.

NOTES:


If the above process doesn’t reveal the Debug menu, then quit the Reminders app and start again at Step 2.
If you ever want to hide the Debug menu, then follow the same process but at Step 2 enter the following command instead:

defaults delete com.apple.reminders RemindersDebugMenu