Sucralose is the most commonly found "pure" sweetener. Although I think you can get stevia as well, if you so desire. I use FA supersweet (I think it is) but most companies will make one. There are some newer sweeteners just being used in
vaping lately. I like sucralose. Some flavoring companies will make a stronger or a weaker version of liquid sucralose. It doesn't need to steep, you can tell how sweet your mix will be (although the rest of your flavors may need to steep alongside it) but start low, you can always add more if needed, and shake it, and vape it, the sweetness part really stays fairly consistent over time.
I am a "reluctant" sweetener and do use some other "sweet" flavors to add depth at times, but if I am using sucralose, it's usually one drop per 30 mls, that kind of thing....
Good luck,
Anna