Comparison to Ring of the Ram
- The same number of charges (3)
- Regains slightly fewer charges (average 2 vs 1.5)
- Both are action to activate
- Less damage (average 11 vs 5)
- Adds str to damage (potentially 10 average damage)
- Attacks with your attack bonus (potentially higher than +7)
- No knockback
- No object breaking
- Melee weapon attack (rather than just an attack)
In the uncommon category, I'm not aware of any item like it. The closest I know of are Circlet of Blasting (7 damage once a day) and Wand of Magic Missile (10.5 damage 7 times per day). We can say that it's probably better than the circlet, and probably worse than the wand.
A paladin could use this item to Divine Smite at 60ft range. There may be other unintended problems, I would be wary of ranged melee attacks.
I'm not sure there is much use case for this ring, a fighter would probably be better off attacking instead of using it.
Comparison to Arrow Catching Shield
- Basically the AC effect but with charges
- Reaction effect can be used on all attacks, not just ranged
I don't think there are many benefits to going into depth on the differences. I can't think of any items that are similar to this at all. Most players would get more benefit from a +1 shield, but it's nice to have a special ability rather than just another option. It is worth noting that the secondary effect being more powerful than the original is very significant. This would be a great item to have.
I think this item would actually fit into rare as much as it would into uncommon.
Combining the two
The versatility of having an item that is both defensive and offensive shouldn't be underestimated. If your DM does put it is uncommon, it would definitely be close to the border of rare.
While I doubt this would be unbalanced, I think you should consider the complexity of the item. There aren't many items that have this level of complexity. Seriously consider making it simpler.
The following is unusually worded: "Make a melee weapon attack against a target within 60 feet." it almost seems to imply you get to make a melee weapon attack which is added to the damage from the shield. Consider rewording to:
While wearing the shield, you can use an action to expend a charge to make a melee weapon attack against a target within 60 feet.
I don't think the requirements are necessary. Stat requirements are not common, and wielding a shield without proficiency already has downsides so most people don't do it.
As I said, the item is basically fine. I don't think it's overpowered, abusable, or confusing. However, I would simplify the item. There is just way too much going on, 3 separate activatable effects make for a fiddly item. Additionally, I just don't see much value in an action dealing 2d4 damage. I set the attack bonus to +5 so it will match CR5 enemies (average AC 14.7, high AC 15.1). So here is my suggestion:
Armor (Shield), uncommon (requires attunement)
This shield has 3 charges and it regains 1d3 expended charges daily at dawn. Whenever an attacker makes an attack against a target within 5 feet of you, you can use your reaction to become the target of the attack instead. If you do so, you may expend a charge to attack the attacker. The attack appears as a spectral elephant and makes its attack roll with a +5 bonus. On a hit, the target takes 2d4 plus your strength modifier as bludgeoning damage.