There isn't a magic additive, PG/VG ratio, or nic source that makes a delicious mix. The actual flavorings and percentages of each are most important, and that knowledge comes with time spent experimenting.
You can DIY juices just as great as any vendor's, if you take the time. In fact, you can do better, because you're mixing to your own preferences. You start with flavors that you personally like, you try each as a standalone flavor to get a feel for the flavor and strength. You take lots of notes. You try different brands of the same flavor to find what you prefer. You experiment with combinations. Great juice vendors have done all that for every flavor in their recipes. Lousy juice vendors add 15% (or whatever standard percent) of Grape to nic base, label it Grape Juice, maybe "reformulate" it later when it gets bad reviews. A great vendor would have tried white grape, concord grape, champagne, etc., know the strength of each, try combinations, maybe recall another flavor that might accent it nicely, try that, get others to try it, all before it's ever sold.
Harsh, soapy, perfumed juices are often from overflavoring. It's easy to avoid if you've taken time to try each flavor separately. A great mix can be ruined by adding one lousy flavoring to it, or too much of any flavor. Lousy juice vendors just don't take the time to avoid all those pitfalls.
And then again, even a popular "premium" recipe might not appeal to you. Even a great vendor who has taken time to develop a popular recipe knows - no matter how great it is, someone will hate it. If you DIY you cater to yourself. You'll never be taken by surprise by a vendor's description of "sweet spicy backdrop" being licorice, when you expected cinnamon. It takes time and patience, but I think it's a lot of fun. The longer you do it, the better you get.