My solution to this whole problem would be as follows.
1) Make Adamantine armor negate crits and adamantine weapons auto crit
2) make the other helmets simply lower the damage taken on a crit. For example you could make the mundane METAL helms (white) do a -10% (rounded down) to all crits, uncommon Helms do -25% (rounded down) to all crits and in the case of Rare Helms (though the only one in the game atm IS the Grymskull Helm which is made of adamantine) gets -45% (rounded down) to all crits
You and I think very similarly. I made a suggestion like that a long time ago. I think the helmets and other armor pieces make little sense in the game. Why would they provide straight Dex Saving bonuses or Strength Save bonuses or completely negate critical hits as if the head is the only place you can be critically hit?
The point of these pieces of armor is to protect against damage. I could see a leather cap offering a +2 bonus to Save versus something like Cold or Fire or both, or a +2 to Save versus Psychic as well. Something like that would make more sense. A leather cap ain't gonna do much good against all types of damage, though, like bludgeoning. I think that's what's really missing from them. They should only offer bonuses to certain types of attacks based on their material and quality, or they should offer a % chance of negating crits, not a definite, absolute no chance of critting ever.
So, for example, you might find a pair of boots that gives you a +2 Athletics bonus to resist Shove, or a helmet that provides a +2 versus Psychic damage. You might find a non-magical helmet that has a 10% chance of negating Critical Hits, potentially turning a Natural 20 that an enemy rolls into just a regular hit, but not necessarily making it absolute that they can't crit you. Pair that with gauntlets of the same quality and design, and now you have a 20% chance of turning a crit into a normal hit. Add boots to that of the same quality and make it 30%. Now you're adding more value to each article and potentially working them together to increase the value of the whole set, AND it only increases the chance of turning a crit into normal, not absolute.
There's SO many things they could do with that to make helmets, boots, gauntlets meaningful and so much more valuable and diverse even. You get one set of gauntlets, "Gauntlets of the Tank" that offer +2 Armor Class. They are strong and even able to endure weapon blows like a shield. Make them heavier, though, and maybe apply a -2 to Attack while wearing them. What good are they then? If you need a person to tank, equip the gauntlets and a shield for +4 AC boost and add your best armor to that. You might not hit well, but if you can keep enemies' attention, you could have like 22 AC or something, and if you're a cleric, you could boost it with Shield of Faith for a whopping 24 AC. Now THAT would make them super valuable. You might not hit well while wearing them, but these are THICK GAUNTLETS.
Another set of gauntlets could be the reverse. They're magical and give a bonus of +2 to Attack but a -2 to AC. Call them "Gauntlets of Bloodlust" or something similar.
There is SO much they could do. SO much.
And back to the adamantine items, yes. That would make sense, then. The armor completely negates crit altogether because it is super armor. The shield maybe provides a +4 AC bonus instead of the standard shield of +2, and it has a chance of damaging weapons that MISS your character by 4 or less (because that's the bonus it provides). So, an enemy attacks you and needs a 20 or higher to hit? They roll a 16. Their weapon is damaged. Better to now roll low against me than high and miss. Anything from a 16-19 is going to damage the non-magical weapon you used against me. (Or maybe a disarm, as you suggested previously since damage might be too OP and they don't have damaging items in BG3 as of yet).
Weapons do auto-crit every time they hit non-magic armor, because they just slice right through standard armor. Yeah. Now THAT'S some serious - worth fighting a raging adamantine golem for it - gear - but you only get one, so what would you prefer? Always critically hitting enemies? Better defense? No Crits?
