Really pleased that you've got that sorted and found a more pleasurable vape. That's what it's all about, and keeping away from the killer tobacco.
As to the flavour and different types of wire / builds, this is a subject that's hugely subjective and can really only be determined through experience. It's all about what works for you. From personal experience, I've used quite a few dual coil devices, but I think you only get the true benefits of using dual coil if you want to vape at high power ratios and put out some serious clouds. For me personally, it's not about whether I can fog up my living room with a few puffs, it's about just having a satisfying vape experience. Therefore, I tend to stick to single coils in most of the devices I use, and I'm more than happy with the performance. As regards twisted kanthal, given the wire has a larger surface area than a single strand of kanthal, in theory this should allow more wire to have contact with the wick and therefore produce more vapor. Have a go with it and see what you think. I've played around with twisted kanthal and can't notice any discernable difference to using single kanthal. Each to their own. It's all down to finding out what works for you personally.
With regards to the subtank rda, I've been using 24awg (0.5mm) kanthal single builds for months, simply because it's a thick wire so I don't have to keep rebuilding the coil every time I want to rewick and change the juice. 5 spaced wraps (spaced wraps work better with high vg) on a 2.5mm bit work out around 0.46 ohm. I could just as easily stick in a different gauge of kanthal, with a different resistance rating, and have exactly the same vaping experience (given I vape around 20w on regulated devices, the resistance rating of the coils is pretty irrelevant as long as I'm within the parameters of what the device can handle).
Good call with the Isle of Custard. Remember their juice is going to require steeping, but I think you'll be blown away with their custards.