Don't mistake that "taking the edge off" affect for "sweetening." That is actually just the beginning of the muting effect. It is smoothing off those sharp "tart" notes and allowing the sweeter notes to come through more noticeably.What I basically wanted to know was if the berry flavor would be tart (like a real berry) or sweeter. A small amount of ethyl maltol could take the edge off the tart and give me a sweeter mix (which I'm going for). I get the temptation to use it as a crutch though. Maybe adding a small amount of a sweeter berry (strawberry?) would be better, I just didn't think to order any because I don't generally like strawberry.
In my opinion, one of the worst myths of DIY, is that EM is a sweetener. Unless one uses it at extremely high percentages (8% and above), it doesn't really add sweetness (and then it becomes a coil-killer extraordinaire). It is (more accurately?) shaving off more and more of the complex notes leaving behind that first/last detectable sweetness that is already there.
In my own mixing experience; I have found that there are much better genuine sweeteners. Sucralose (CAP-Super Sweet, TFA-Sweetener, etc.), and DIY-ready stevia (OSDIYS, NN, etc.), add a much more concentrated, truer sweetness to a mix. Which, encourages using less (and therefore less gunking), and has less of a muting affect.
The use of secondary/accent flavors, like you suggest, is a a great choice. The use of light(er) flavor concentrates; like dragon fruit, pear, raspberry, strawberry, watermelon, or grape, at low percentages, can add a subtle sweetness without creating a noticeable presence of their own. Which of these is best, for any given mix, will depend on the recipe focus. Pear and dragon fruit are almost always good choices; the others can be a bit more recipe-dependent.
As always, all of this is only my opinions; based on my own mixing experience. Other mixers may have different experiences and opinions, which are every bit as valid as my own. I would encourage you to try different ideas/methods and see what works best for you.