RAW, yes Barkskin *should* set your minimum AC to 16. It should NOT stack with anything else; if you have an Unarmored Defense of 18, then with Barkskin your AC will still be 18.
That said, Barkskin is underpowered imo mainly because it requires concentration. It has such a low effect (increasing your AC by 2, maybe), such a high opportunity cost (can't cast other concentration spells), and can so easily be lost if you take damage. Additionally, either your druid is ranged (and thus probably doesn't need additional armor, and certainly wants the concentration slot for a better spell) or is melee (in which case AC 16 still isn't that great, and you'll likely lose concentration in 1-2 rounds). So I wouldn't be opposed to a buff...
-2 or -3 should be still acceptable. -5 is too steep for me. does larian follows monster CR? as i said if GWM is good for low level monster it wouldn't be useful for me. usually we wont have problems with low level monster anyway. in solasta even with advantage the miss chance is high too.
one noticeable enemy is warlords. if not mistaken having like 22 AC if not mistaken. i have like +10 to hit. to hit a warlord even with advantage you need to roll a 12 and above which is not so easy. such high AC i find using phantasmal killer much useful.
Are you suggesting that GWM should be -2 or -3 for +10 damage??? Or -2 for +4 damage?
The only Warlord statblock I'm finding is AC 18 and CR 12, meaning that characters should be level 9+ (attack bonus of about +9) when facing it. That's a 60% chance to hit which is not bad. GWM with Advantage still wins unless you do >13.5 (>1d12+7) average damage on a normal hit.
