This is an improvement on the 1st level feature Wrath of the Storm
The Tempest Cleric’s 1st level feature, Wrath of the Storm says:
Also at 1st level, you can thunderously rebuke attackers. When a creature within 5 feet of you that you can see hits you with an attack, you can use your reaction to cause the creature to make a Dexterity saving throw. The creature takes 2d8 lightning or thunder damage (your choice) on a failed saving throw, and half as much damage on a successful one.
You can use this feature a number of times equal to your Wisdom modifier (a minimum of once). You regain all expended uses when you finish a long rest.
The 6th level Thunderbolt Strike feature synergizes well with this feature, allowing you to deal damage and push the creature away from you with your reaction. You can even use channel divinity to make it deal 16 damage.
This can be especially useful for kiting creatures with multi attack. If a creature expends all of its movement to reach you, then hits you with its first attack, you can use your reaction to push the creature out of reach, so that it is unable to make any further attacks.
Since class features do not assume you will be given any magic items or play a certain race, it seems this is the canonical intended function of this ability.
Ask your DM if you can swap some spells on your spell list.
If you want more flexibility here, I recommend just working with your DM on swapping out some of your existing spells for more lightning damage spells. The DMG says this is okay:
Modifying a class’s spell list usually has little effect on a character’s power but can change the flavor of a class significantly. In your world, paladins might not swear their oaths to ideals, but instead swear fealty to powerful sorcerers. To capture this story concept, you could build a new paladin spell list with spells meant to protect their masters, drawn from the sorcerer or wizard lists. Suddenly, the paladin feels like a different class.
This shouldn’t make your Tempest Cleric feel like anything other than a Tempest cleric. It just gives the cleric more of what it should have had to begin with.