Seems fine

Using an action to do 2d4 + strength bludgeoning damage isn't all that interesting for a level 5 character. If you have 16 strength or more, you are most likely a class that has two attacks at level 5, so your option is something along the lines of doing 2d8 + 2*strength at melee range if you just hit normally with your two attacks, or doing your 2d4+strength at range. The only real advantage here is that almost nothing will resist magical bludgeoning damage, but that can also be accomplished by having a magical weapon.

Using your reaction to gain 2 AC if you would get hit, if you can see the attacker, is pretty minor. It'll save you at best 3 hits per day (because your charges are limited), which while useful, is only slightly more likely to make a difference between a hit and a miss than a normal uncommon + 1 shield, which is active 24/7. You might just want to buff this to allow you to use a charge to cast Shield as a reaction, which gives +5 AC. That way, it's a lot more likely to be a meaningful choice.

Only the third ability seems like it could be considered problematic. By not requiring charges, this would allow a high AC tank to stand next to a squishy target, and once per turn take a hit in their stead, which is extremely strong. Compare this to class abilities like a Cavalier's level 7 ability Warding Maneuver or a fighter's Protection fighting style and it is most likely equal or better than those two abilities. If nobody in your party has those abilities, it should be fine, but it can feel kind of frustrating if an uncommon item does what your character is supposed to be doing better than you, so if you have one of those kinds of characters around, this one might require tweaking.

All in all, it seems like a pretty balanced item. The third ability is really the make or break here, if the party has another melee fighter who's very squishy, the ability is really good. If the rest of the party is all ranged characters, it's most likely a lot less interesting and your players may decide that a + 1 shield is a better choice in the long run, depending on how many fights they have in a day.

This is profoundly unbalanced as written. During downtime days, as written, the item permits the user to pour unlimited numbers of hit dice into the bracers. All you have to do is ensure that you don't end the day with less than half your maximum number of hit dice, and that you finish storing at least an hour before you go to sleep, and you're losing nothing for the next day. You can then tap those whenever you like for major blocks of healing and/or curing the blinded/deafened/paralyzed/poisoned conditions.

The cost of storing is meaningless because no one's going to take the Store action while in combat anyway. There's no reason to, and actions in combat are far too precious to spend unnecessarily. If they're safe enough to store, then the couple of rounds won't matter. Spending a die to cancel the debuff is if anything even more pointless, because then you're burning the die you just stored in order to get rid of the (minimal) side effects of storing it. Between that and the action cost, it means that you're effectively burning a hit die and two actions in order to give yourself a durability penalty for a round while in combat. It's just a terrible idea all around.

In order to have any hope of being balanced, there needs to be a serious limit on how many hit dice can be stored in the thing. Either there's an explicit cap, or the hit dice that are stored in the thing simply don't refresh while stored. The way that gets handled is going to be a significant part of how balanced it is and for what rarity. When the answer is "there is no limit", it breaks the HP economy and therefore cannot be balanced. None of your proposed fixes address that core issue in any meaningful way, and therefore none of them are particularly pertinent here.

For where to put the hit die cap, if you want to go that route? Well, the ring of spell storing is an attuned rare item (a very nice attuned rare item) that can store up to 5 spell levels in a somewhat similar way. Those spell levels could be healing spells (but generally aren't). Figure out how much healing you could reasonably get from packing one of those things full, and that's a floor... because the ring of spell storing is way more flexible than that. For another benchmark, compare it to the max cap of the staff of healing (using only 9 charges rather than 10, because you don't want to risk a 5% chance of it just vanishing). That, too, is an attuned rare item, and its ability to heal others makes it somewhat more flexible, but it's getting closer. The "staff possibly vanishing" and "only cleric/druid/bard" aspects bring it down a bit.

As a sidenote, by my understanding of Brandon Sanderson's work, this sort of thing may be a recurring problem with trying to convert his ideas into magical items for 5e. 5e is pretty careful about not letting things break the system. Sanderson is all about having people take the powers they have access to and figure out ways to use them to break the system. The two philosophies don't mesh particularly well.