Do you mean unflavored? Because naturally flavored JUST means that the initial base flavoring is made from "found in nature" not even necessarily "banana" but say, "Chicken dung chemically addressed and modified until it tastes like banana, even." OK maybe a touch of hyperbole but seriously, natural flavors are NO less chemically modified, changed, and converted than artificial flavors. Flavor labs are flavor labs and they all do exactly the same thing whether it is to a strawberry or "Chemical 599&XY." Natural flavors exist SIMPLY because they sound better, so capture a higher market price. They are no "healthier, " and I'd wager a guess sometimes LESS heathy than artificial flavors. Flavor labs are not in the business of looking out for your health.
You could try "organic" flavors although again, there is a certain amount of hucksterism and since eliquid is really low on the regulatory spectrum (for now) I am not sure you'd have any sort of 100% certainty regarding how it is made, and what chemical processes it undergoes, but I can pretty much guarantee you'd get like, a price hike for going to that "market area."
If you try unflavored, that might be the smartest way to go. For one, it's not terrible, especially if you add in some of the "nuttiness" that unflavored WTA has to it. It's not terrible tasting.
Secondly it is entirely possible that flavoring is not your issue at all. I became "sensitive" to PG after a fair amount of exposure, so your first "heavenly" batch of flavored juice may have been because you were not sensitized to other things in eliquid and your best bet for discovery of that is to go to as few ingredients as you can (and I might include different nic levels etc..)
Also, brain fog can be part of detox, you may have just hit the part of detox where brain fog became prominent for you. If that is the case, I would expect WTA to be helpful in that aspect, as it retains some of the alkaloids in tobacco.
If you start with unflavored 0 nic VG, you will have a baseline, then start adding in Nic, PG, finally WTA, whatever.
The good news is, once you have found your unflavored "stew" that works, you can start adding in flavorings as you go, to see what you are sensitive to or NOT, just like an allergy elimination diet.
Finally. Did you repurchase your first successful juice to see what happened? That would have been like, the FIRST thing I had done, because if that juice remained "okay" I would have given myself a TON more information about whether it was actually the flavorings or NOT.
Good luck though. Honestly, before you do all the rest of it, you might want to just repurchase that initial flavor nothing to lose there really.
Anna