I quit smoking using 3mg juice. From experience, higher nic juice will make it easier, it took a lot of willpower on my part to stick with it on 3mg at the beginning. I would suggest 6 or even 9 mg juice and start cutting it back slowly. The only reason I used 3mg only is because I had a lot of it stockpiled from when I was dual using. It also let me chain vape to help keep my hands busy, which was the hardest part of quitting for me. After 20 years of having a cigarette in my hands, I just had problems breaking that physical habit.
Take the transition from smoking to vaping at your own pace. Nobody here is going to give you better feedback than what your body is, so listen to it. If it is telling you it is needing more nic, you should probably listen and increase your nic level to the point that you don't constantly crave. If your feeling bad, you may be over doing it (which is easy to do when dual using, and also makes it harder to know if your ejuice is strong enough for you). Take it slow, and if you feel panicked, set down and take a couple draws from your vape and calm down. We have all been there, it is the hard part, but once you get over the hump, you will feel much better, and feel better about yourself.
Thank You