As an alternative, you could just let Clerics help a downed ally as a Bonus Action kind of like how several classes can Hide as a Bonus Action. It isn't particularly powerful and makes Clerics a bit more interesting. You could also take Prayer for Healing off the Cleric Spell list and just make it an ability that is used automatically when you take a Short Rest (in 5e, it was a 10 minute cast time). Prayer for Healing is kind of useless as it stands because you can just go to camp when not in combat anyway; this change would make it more likely your party would venture just a bit more before taking a Long Rest with no real impact on power level.
I'd give Clerics Prayer of Healing as a 1st Level ability that heals everyone Cleric Level x 1d4 + Spellcasting Modifier hp with each Short Rest and remove it from the Spell List.
I'd give Clerics the Bonus Action "help a downed ally" at 2nd Level.
With an additional offensive Cantrip, that might be enough to make them interesting enough to increase popularity for the class without increasing power much.
You might also consider taking away some armor for more interesting and/or thematically appropriate abilities. Like Nature Cleric could have Medium Armor (instead of Heavy) and Proficiency with Nature and Survival. Knowledge Cleric could have Light Armor (instead of Medium) and Proficiency in all of Arcana, History, Nature, and Religion and Expertise in two of them (instead of just Expertise in two of them). Trickery could have Light Armor (instead of Medium) and Proficiency with two of Sleight of Hand, Stealth, and Deception (plus the other could be chosen as one of the two starting Cleric Proficiencies). Not more powerful, just more interesting.
