But does that really feel like Barkskin? That's my secondary complaint about the spell (the first being the odd drafting). It doesn't make sense that a character with tough skin wouldn't benefit from a shield or from ducking behind a wall. Intuitively, tough skin and cover should stack. (And doesn't a focus on naturalistic rules means that we should be able to trust our intuition?) Giving a base AC when you have nothing else going for you feels more like a luck or divination spell, not a transmutation.Perhaps:
"This spell magically protects you when everything else fails, granting you AC 16. If your AC from regular sources (including, but not limited to, Dexterity modifiers, armor worn, shield bonus, and conditional modifiers such as cover) would be higher, use that AC instead."
I'd write it as
"Your skin becomes unnaturally tough and rigid, like the bark of an ancient hardwood. Functionally, your skin acts as chainmail armor, giving you an AC of 16, and you no longer gain a bonus (or are penalized) for your Dexterity modifier to AC. This armor requires no proficiency, and you do not suffer a penalty to your movement for having a low Strength, as with standard heavy armor. Just as with chainmail, you suffer a disadvantage to Stealth checks."