Is this homebrew entropic spear balanced?

Question

I have created a magical entropic spear that I plan to give to the party when they reach roughly 10th level. Is this balanced?

Entropic Spear

Weapon (spear), rare

This is a +2 magical weapon. On a critical hit, this spear does an extra 2d6 damage of a random type. Roll a d10 on the table below to determine the type:

1. Acid
2. Cold
3. Fire
4. Force
5. Lightning
6. Necrotic
7. Poison
8. Psychic
9. Radiant
10. Thunder

This spear has 5 charges, regaining 1d4+1 charges at dawn. As a reaction, when an attack hits you, you can expend 1 or more charges to reduce the damage by 1d6 per charge spent. You can only reduce damage of a type that appears on the above table. If you use the last charge, roll a d20 after applying the effects of the item. On a 1, the spear crumbles into dust and cannot be repaired.

Answer 1

It is balanced

Its magical properties are a bit better than a vanilla +2 rare weapon, but this can only be a spear.
Most characters would do higher DPR with a +2 Longsword, so I do not think it is too strong.

Answer 2

It's balanced, but you could make it very rare

Most characters by 10th level will have a +7 or +8 bonus to hit with regular weapons. Given that this spear gives an additional +2, it would give them a good hike on the bonus to hit, making it so that most creatures they confront would be hit 55-60% of the time. You also add the equivalent of a greatsword hit in damage die, but at least the type is random. It's properties are, I think, well offset by the limited number of charges.

Overall I think this is a balanced item, but you may want to make it very rare instead of rare.