My Experience with Ranged Smiting
I have created a magic item for my players that allows for ranged smiting. They are magical arrows that cost gold (I give my PCs the ability to craft magic items), and they deal smite damage on hit. In exchange for ranged smiting, however, the paladin must declare the smite and expend the spell slot for the smite before they fire the arrow. If the attack misses, the smite is wasted.
Why I like this compromise
I feel like this creates a useful middle ground for the paladin. In play, she still goes for the melee attacks most of the time; only when the enemies are truly out of reach does she turn to the arrows. In effect, you are balancing the incredible versatility of a ranged smite with the possibility of wasting a spell slot, turning the smite into a ranged attack roll. Moreover, because firing the arrow requires DEX, your paladin has to make the same spellcasting stat vs. attack stat decision that they normally do. Finally, because they are consumable magic items, you can give out fewer of them if you think they're too strong, and there's an aspect of resource management to them as well.
Also, this makes sense in-universe: the Paladin can choose to use the smite from her weapon because she's always holding it, but if she shoots an arrow, she can only infuse it with the smite while it's in her possession.
How this applies to your situation
Unlike your situation, my party was not all paladins. However, it's clear that even within your party, there are distinct specializations, and not all paladins will be able to benefit from ranged smiting. A dex-focused paladin would benefit far more than a str-focused paladin, for example, and it might feel unfair if the ranged paladin can do everything the melee ones can, but be out of harm's way as well.
Since your situation is unusual, I'd suggest phasing in smite arrows slowly. At first, you can introduce arrows that can only deliver 1st level smites. If those work out with the party, you can introduce arrows that are capable of higher level smites, as well as the smite spells. Ranged smites too powerful? Then give fewer smiting arrows. The ranged paladin's too weak? Then give more powerful arrows. Because the arrows are consumable magic items, you can easily "titrate" the ranged smite ability you give.