Quick way to sync now
Note: This answer is not tested on Android Marshmallow.



In Android 5.0, one can create shortcut for sync using the activity com.android.settings.Settings.$AccountSyncSettingsActivity (since I've only one account in the whole device, the activity may do nothing for multiple accounts without passing specific parameters).

Anyhow, attempt to create the shortcut for sync using an app like QuickShortcutMaker or Activity Launcher or even an automation tool like tasker. 

As per OP, it has  come to my notice that sync shortcut does nothing in Marshmallow. OP also mentioned that the activity for Accounts works well. So for the time being, settle with Accounts activity (com.android.settings.Settings$AccountSettingsActivity). The shortcut can be created using the aforesaid apps.