Dipping into a level of cleric is my intention for religious paladins, but I confess I thought the paladin-specific lines were just leftovers from an abandoned earlier plan to let paladins choose deities (which would have been great). I’m not sure they’re there consistently through the game … though I’m not sure they’re not either. I did complete the game with a cleric of Oghma/ancients paladin character but clearly wasn’t paying enough attention! But even if there aren’t paladin-specific options, I found the cleric ones good enough to give the right flavour to my character.
If I want to do this, how many cleric levels should I get before switching to paladin? I'm inexperienced with multi-classing in BG3.
If doing this, it can be better to take paladin level first to pick up armour/weapon proficiencies. I’m not sure how BG3 manages these for multiclassing, as you do seem to get quite a few proficiencies from new classes, but I’ve just checked and a nature cleric > ancients paladin doesn’t get heavy armour proficiency whereas other way round they do. Though taking a level of paladin second does give martial weapons proficiencies, so if choosing a domain that gives heavy armour you might be okay.