Definitely agree there’s an imbalance here. I found I just stopped using Holy Rebuke after my initial experiments with it. It’s unlikely that there’s ever going to be no better action a paladin can take.
There are various ways it could be tweaked to make it usuable. Possibly turning it into a bonus action like Healing Radiance, or even a reaction sort of like Hellish Rebuke (though triggering if an ally were attacked, rather than the individual with the power). Though it would probably still become obsolete pretty quickly if it continues not to scale with level or, say, charisma modifier.
