Yes, good suggestions; I'm not really familiar with D&D lore though...do beholders actually get along with other monsters? Do they have servants?
Not sure if lower AC bonus would have that much of an effect, after all you can easily switch to a shield with high ac bonus if you fight against dangerous melee opponents, so you'd have to give beholders a powerful melee attack as well (do they have that in D&D?).
But those beholder caves (iirc there are two in the game) really aren't that great...either a breeze with that overpowered shield, or extremely difficult. I usually did only the important quest-related stuff in them, killed the beholders that were in the way for that and left the rest alone.
Another item I really disliked is the cloak of mirroring...that one was laughably overpowered as well.

Making the shield cursed would make it so that you *can't* easily switch; you would need to cast a specific spell on the character (one that currently goes unused by the vast majority of players) in order to switch.

I really think the game should have had some useful cursed items. Some of the more powerful items (like the Robe of Vecna) should have been cursed with some negative effects (so that equipping such items isn't an automatic choice) and some of the cursed items (at minimum, those with unique names and descriptions) should have been made powerful enough to justify their use. Perhaps if the Backbiter spear would give its wielder a nice backstab multiplier (even if the attacker isn't a thief) and a decent damage boost, it might be a worthwhile weapon to use, especially on a character with enough HP for the curse damage to not be life-threatening.

Wizardry 8 had the right idea with cursed items; many of them (Staff of Doom and Bloodlust for example) are quite useful if you are willing to deal with the curse. I find it rather disappointing that Baldur's Gate 2 seems not to have any items of this sort.
I'm thinking that maybe a fix would be something like this:
1. Make the item cursed (again) so it can't be removed without Remove Curse
2. Make it damage the user over time (so you are definitely *not* invincible)
3. Make it reflect beneficial spells (but not Remove Curse) back on the caster

Making the items cursed might have been a good idea...I think some negative side effect would have been good (like lowering a character's stats as long as that shield is equipped)...though I'm not sure if that would have been enough to counterbalance the shield's power.
Robe of Vecna...that's the item that reduces casting times iirc? That one was also pretty ridiculous, don't think I ever used it for a real playthrough.
They shouldn't have added those bonus items in the Adventurer's mart...BG2 had far too many magic items as it was, and those bonus items were just overkill (the Planescape Torment connection of some items was also rather tasteless).

Lots of things are imbalanced in Baldur's Gate II. Once you learn what's good and what isn't, it's really all just self imposed challenges or tactic mods.