If it were my game and my players asked me?
I'd replace Divine Smite with Divine Ward (similar to the Arcane/Projected Ward of the abjurer). It is a no-action ability powered by spell slots (similar to Divine Smite). Really, you're trading "decrease bad guy hit points" for "increasing good guy hit points", which, IME, is roughly the same.
Divine Ward: Starting at 2nd level, you can create an aura of divine magic around yourself for protection.
When you hit an enemy with a melee weapon attack, you can simultaneously expend one paladin spell slot create a divine ward on yourself that lasts until you finish a long rest. The ward gains hit points equal to 2d8 for a 1st level spell slot, plus 1d8 for each spell level higher than first, to a maximum of 5d8. The hit point total of the ward is limited only by the number of paladin spell slots available to fuel it.
Whenever you take damage, the ward takes the damage instead. Your melee weapon attacks do an additional 1d8 damage versus an undead or fiend while the Ward is active [OR: If the damage is from an undead or fiend, you may reduce the damage by an additional 1d8 before subtracting damage from the ward]
If this damage reduces the ward to 0 hit points you take any remaining damage. While the ward has 0 hit points it cannot absorb damage, but its magic remains. You may recharge the ward at any time by expending one paladin spell slot of 1st level or higher. When you do so the ward gains hit points equal to 2d8 for a 1st level spell slot, plus 1d8 for each spell level higher than first, to a maximum of 5d8. Once you create a ward, you can't create it again until you finish a long rest.
Starting at 6th level, when a creature you can see takes damage, you can user your reaction to cause your Ward to absorb that damage. If this damage reduces the ward to 0 hit points, you, not the target creature, take any remaining damage.