Healing in a classical rpg sense is not that strong or even possible in BG3. You should rely mainly on potions on all characters for that. It's usually a waste to make one char throwing potions. The most important task for a Cleric in early and mid game is to buff with Bless, especially if you have the staff from the mage tower in the underdark. Healing Word in case of downed companions is very strong, one of the best spells in the game. It's not necessary to talk about Sanctuary, it speaks for itself.
Later it comes in very handy that you have access to several cleaning spells as Cleric. It is an advantage of Cleric and Paladin that you get all spells without learning which can be changed from the Spellbook. I think lately my group would have been wiped (bad thing in Honour mode) because of a special curse, had not my main scouted the area alone, so that Shadowheart was not in the fight immediately and could change her spells. After that event "Remove Curse" will never leave my spell bar again.
In the end I don't think multiclassing a Cleric does make so much sense. Several classes win by dipping into Cleric, but as main Cleric you can and should go 12 lvl. The good minions are from a level lvl 6 spell you only get as full Cleric. The better animated undeads need a lvl 5 spellslot, so having two is good.
If multiclassing I would think about the leveling effectiveness. If you start as Cleric you get proficiency in Wisdom saving throws, quite important and much better than what a Fighter gets. However Fighter gets more feats. I would always go to Fighter 6 first if Fighter were an important part of my spec.
