Welcome to the hobby of mixing
I have used many methods for mixing and steeping and have found varying results. Right now my favorite way is with a magnetic stirrer that heats up. I heat most liquids up until the vg thins, and certain flavors upwards of 160 F degrees, even 180 F in some cases. Not all juice responds well to heat.
However, even though I mix on the magnetic stirrer, I still do the old stand by of shaking and setting up on the shelf to let time do it's thing. When creating new recipes I will mix two batches, one on the magnetic stirrer and one just shaken and left to steep. I do this to test flavor reactions to heat and stirring. For flavors where heat works better, it is often due to a chemical note in a flavor or alcohol in the extracts.
Magnetic stirrers aren't cheap, the one I bought was about $200...though it can be found sometimes used for around $100. In most cases you can apply gentle heat (hot tap water, crockpot on warm) and leave your juice open to allow it to breath to remove some off notes. Cover with a cheesecloth if you are concerned about dust. Shake the liquid with the lid on, and return to the heat until you are satisfied the off scents have dissipated. Steep for up to 4 weeks before deciding how bad a juice is, and even some juices can take up to 6 months to fully mature into something amazing... most of us experienced mixers have had at least one mystery mix in a cabinet for 6 months+ -- pull it out and are just blown away at how good it is. But then you know you didn't label it and and don't have the recipe to an amazing juice that took forever to steep. smh.
I generally try juices freshly mixed, then 48 hours, then 1 week, then once a week until I am satisfied with the steep when I normally end up vaping the whole bottle.
Sometimes there are some recipes that even with steeping can't be saved. I've mixed plenty of drain juice over the years, and usually it's how certain flavors blended together in the mix. Sometimes I can pinpoint the culprit, and adjust the recipe, but sometimes it's just a lost cause. I've abandoned lots of recipe ideas because the available flavorings don't taste how I want them to taste.
Mixing test samples of all new flavors as single flavor juices at different percentages (normally I mix 10ml or 30ml single flavor samples.) I'll look around online to see what the average usage for that flavor is as a single flavor, then I will mix it at average % and then two more bottles with a higher and lower %. This gives me a range, and normally I can tell how the flavors will behave with other flavors at different percentages this way. It took me a little while of trial and error with this method, but once you start doing it, knowing what % to mix a flavor at based on how it smells in the bottle will start to come easy.