Legendary Resistance as a reaction is interesting.I think this is the closest to what I think might be a good solution. Move Legendary Resistance to the Legendary Actions/Reactions section. Using the Resistance uses up one of its actions for the round so it has less offensive thing its doing. Using the Resistance might have a "backlash" effect on more powerful creatures, sort of like a Hellish Rebuke. For example, a Beholder's Legendary Resistance when it fails a spell save is that it instead makes the save AND the anti-magic field from its central eye refocuses on the person who casts a spell on it.