In my opinion turning up the wattage/voltage doesn't always induce more flavor. It depends on the liquid itself. I have some liquids that get really vacant tasting the more I turned up the watts, and while providing less vapor produced a far more pronounced flavor at 7.5W than it did at 9 or 11W. Obviously taste is subjective, I have a neighbor that just got into vaping and she was raving about some liquid she put in so I had to give it a try. I immediately started gagging when I inhaled on her PV as it was dry hit city. I ask her to try it again and she did and no response, no gagging, she loved it. But I swear it was burning cotton.
Anyway, it depends on the liquid, some are more flavorful at lower wattages and some at higher wattages.
Your MVP should be both variable wattage and variable voltage. Since we usually ditch our coils before they get too bad I don't see much of a difference in using watts vs. volts since if I understand it correctly, you set the unit for watts and as your coil gets old the resistance goes higher so the variable watts keeps consistent with that where as the volts just stays consistent at the volts you set it to.
As far as the coil changing, yup I used to change mine anytime that I changed liquids, but if the coil was still good I'd just stick it in a ziplock baggie and write down what liquid was used in it and the date so that when I wanted to go back to vaping that liquid I could use the old coil (yeah, I'm cheap skate). You can use the same coil but only if the flavors compliment each other, like I was vaping something that is supposed to be like a maple syrup, then I ran out of that liquid and was too lazy to rewick my subtank so I just stuck in some salted caramel and it worked out great, I tasted the syrup for a short while and then the caramel and maple and finally now it's just caramel only.
I think the ones that I would change the coil every time would be any of the space jam juices, they're hellishly strong tasting and doesn't matter if I soak a coil in vodka for a month, that smell is not coming out of it. I've even made it so that any of the space jam juices only go into certain tanks that are reserved just for that, just like my needle bottles, some are reserved exclusively for their juices because the smell of them simply won't go away.