If you're looking for the sweetener that also mellows strong flavors, you want diluted ethyl maltol, which is sold as "Cotton Candy" flavoring. You'd add maybe a drop or two to a 5ml bottle of premixed juice to take the edge off...or as much as a couple of drops per ml of premixed juice to sweeten considerably and mute the harshness. The other sweetener you'll find premixed is Sucralose-based, and that's probably not what you want. Sucralose is much sweeter and does not mellow the flavor of what you put it in.
If you have a harsh premixed juice, you can also mellow it out with a cream flavor. You can mellow it and add sweetness by combining it with a premixed sweet vanilla, caramel, or other flavor you like.
And I can't resist....my favorite sweater is a wool cardigan.