Give the Caramel Cap another chance. That was my first flavor from boB & Missy. At first vape I thought "yuck." I decided to try it again the next day before trading it and I finished the whole bottle very fast.
The most common way to sweeten e-liquid is with liquid sucralose (aka Splenda.) Unfortunately, I've developed an allergy to it over the years so I'm getting mine sucralose-free. KBV uses sucralose in some of their liquids (I don't know which ones) so getting a little extra might be an option in the future. On the other hand, once you get good at it, you might like to control the extra-sweetness level yourself.
Stevia - I'm assuming you bought it in liquid form. Is it in water or PG? I haven't had luck with the water-based versions which taste like pine tar to me when the atomizer starts getting dry. My PG-based one is a little stale but did work better.
Flavoring - I have Cotton Candy flavoring (malitol) and ordered some Brown Sugar flavoring that I'm playing with instead of sucralose. perfumersapprentice.com has both as well as liquid sucralose.
How much to add is a trial-and-error thing. Don't just pour them into your entire order. You can't get it out again if you added too much. Wash out an empty joose bottle and try a mix. Adjust it and try again.
And, if you are looking for sweet, try Gooey Better Cake. Without the sucralose, it is already incredibly rich and sweet. Maybe the real cake is sweeter but, as I've never tasted it, I have no way of knowing.