Is this homebrew spell extension to the shield spell balanced?

The spell is supposed to be a more powerful version of the shield spell that produces a shockwave back to the attacker when the enemy hits the shield. As such, it’s supposed to follow the same mechanics as shield with just some additions.

Energy Shield

3rd-level abjuration

Casting Time: 1 reaction (which you take when you are hit by an attack or targeted by the magic missile spell)
Range: Self
Components: V, S
Duration: Instantaneous

An invisible barrier of magical force appears and protects you. Until the start of your next turn, you have a +5 bonus to AC, including against the triggering attack, and you take no damage from magic missile or lightning bolt.

* - which you take when you are hit by an attack or targeted by the magic missile or lighting bolt spell

When an attack misses due to the AC bonus:

• An energy shock erupts from the energy shield back to the attacker, dealing the original damage of the original attack and knock him prone on a failed Str save against the spellcaster's spell save DC, or half damage on a successful save. This does not apply to ranged attacks.


• The energy shock stops the weapon and delivers shock damage due to the force of impact. Subpar weapons automatically break. Standard weapons have a 20% chance to break on impact. Magical items are not broken.


• If the attack was a magic missile, the magic missile is rebounded randomly to another nearby creature (including friendlies) or the original attacker. Lightning bolt simply dispels.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the duration of the spell is increased in one turn per level above 3.

Clarifications

• 
  Subpar weapon: At the discretion of the DM (i.e. the rusted sword an orc is using)


• 
  Original damage: The roll the original attack was supposed to make. It's made as if it was a hit but the results are applied to the original attacker.


• 
  Missed due to the AC bonus: Say your base AC is 12, spell increases it to 17. Any roll that falls between 12 and 17 is considered to "hit the shield" and therefore trigger the energy shock, while bellow 12 is considered a miss (didn't even hit the shield at all). Note since this is triggered as a reaction to a hit, this will apply to most initial attacks (assuming the +5 AC increase is enough to stop the attack). The range is in place for subsequent attacks until next turn of the player (or in higher level until the spell ends)

If it's not balanced, could a change in the base level of the spell fix it?

No, the spell is not balanced; it is both overpowered and overcomplicated.

Let's break down the spell's features (I'll provide a rebalanced version below):

• AC bonus is identical to the Shield spell, so no issue here


• immunity against Lightning Bolt: if this is what you thematically want for the spell, it should not be an issue, balancing-wise. However, if you want to stay more in line with the original Shield spell, choosing another spell that deals force damage might be more appropriate (see D&D Beyond for a filtered list of spells)


• "When an attack misses due to the AC bonus": this is unusually specific for 5e, although the DMG has a variant rule on "Hitting Cover" (see DMG, p. 272). I think it's not too complicated yet.


• the energy shock: I think this is a little overpowered, and it also creates issues with Lightning Bolt - which is a line of effect, and you might not be the only target (another reason why it might not be the ideal spell to add here).
  
  Let's take Hellish Rebuke as a comparison. It also requires your reaction, and deals 2d10 fire damage on a failed DEX save or half on a success. So, currently, you would have to cast two level-1 reaction spells (which is impossible, you only have 1 reaction) to get the benefits of shield and deal 2d10 fire damage to the attacker. Your energy shock allows the benefits of a Shield spell, and also deals damage back to the attacker and knocking them prone. Off the top of my head, the damage dealt by the creatures you fight at level 5 (which is when you first gain access to 3rd level spells) should be roughly equal to 2d10. However, this increases at later levels, increasing the benefits significantly. Basically, your spell scales without requiring higher level slots, and it's strong to begin with - depending on how far the creature has moved to get to you, they're likely not able to get up from being prone more than once this turn (if at all).


• breaking weapons: just don't. 5e (and probably also previous versions) goes to great lengths to avoid affecting player's equipment. Take, for instance, the Fireball spell, which would logically incinerate your clothing and be a huge pain in the butt for any archer, since their bowstrings burn up. Because this would ruin the game for most people, the spell includes a phrase "It ignites flammable objects in the area that aren't being worn or carried", explicitly not harming creatures' equipment.
  
  You, on the other hand, are actively adding a feature that explicitly breaks equipment, and is prone to DM adjudication concerning the definition of "subpar" weapons. Since you're listing an orc's rusted sword as a subpar weapon, I'm assuming that you're thinking of players breaking their enemies weapons with this spell. However, logically, any enemy spellcasters should be able to maybe have the spell as well, and then annoy your players' characters. Keep that in mind.


• 
  Magic Missile rebound: this should be ok, since it can also hit your allies, but considering the overall complexity level, I would leave it out.


• 
  Lightning Bolt "dispels": this is good, anything else would be overpowered. However, you should avoid the keyword "dispel", since that hints towards Dispel Magic, and Lightning Bolt cannot be dispelled. Also, I don't feel like your spell is meant to completely negate Lightning Bolt, including against other targets ...?
  
  I think "Lightning Bolt has no effect against you, but is not redirected either" or a similar phrasing would be better.


• Upcasting: this is where the balancing gets really broken. With a 5th level spell, for instance, you have an incredibly strong feature for 3 rounds that only requires your reaction on the first round. This is by itself already broken, but it also means that on subsequent turns, you can also cast Shield, accumulating a +10 AC bonus (since they're different features).

So, with all of these issues in mind, let's take a look at my revised version of your spell:

Energy Shield

3rd-level abjuration

Casting Time: 1 reaction*
Range: Self
Components: V, S
Duration: Instantaneous

* - which you take when you are hit by an attack, targeted by the magic missile spell or when you take force damage.

An invisible barrier of magical force appears and protects you. Until the start of your next turn, you have a +5 bonus to AC, including against the triggering attack, you take no damage from magic missile, and you have resistance against force damage.

When a melee (!) attack misses you that would have hit without the AC bonus gained by this spell, an energy shock is triggered against the attacking creature. This energy shock deals 3d8 force damage to the target, and it knocks the creature 5 feet away from you. In addition, the attacker drops his weapon in his space.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the damage dealt by the energy shock increases by 1d8 per level above 3.

In addition, if you really want to keep the multiple-turn-duration, you should add a clause that makes this spell unable to stack with the Shield spell, and the duration should be increased by one turn every three levels. For instance, a level 3 Energy Shield lasts until your next turn as normal, while a level 6 Energy Shield lasts one additional round, and a level 9 Energy Shield lasts two additional rounds.