Cleric/Paladin multiclass

[5eNeedsDarksun]

Which is exactly what happens with this NPC in the actual BG2 game... He remains a Fighter-Cleric dual class, but just adopts the title of a knight. He doesn't actually transform into a Paladin, but continues in his Cleric progression.

Therefore, the most accurate portrayal of this NPC in 5E would be something like a Fighter 5 or 6/Cleric X, with the knighthood simply being a social/personal thing with no bearing on his class. He'd simply continue taking further Cleric levels as before.

And I disagree with the OP's assertion that a Fighter/Cleric "doesn't work so well in 5e"... Fighter 5 or 6/Cleric works just fine in 5E for a Warrior Cleric who wants to focus more on weapon attacks than full casting. In return for your lesser Cleric casting ability, you get access to Action Surge, and a Fighting Style, plus a fighting ability like the Battlemaster's Maneuvers or the Champion's Improved Critical. But you're still able to have some secondary healing and buffing, and you'd still a better caster than either a straight class Paladin or especially a Cleric/Paladin by Tiers 3 and 4, while still being a better weapon attacker than a single-classed Cleric in all Tiers. All without having to delay Extra Attack at all. (Or by only one level if you do something like Cleric 1 -> Fighter 5 -> Cleric X, to get a little Cleric spellcasting online in Tier 1.)

Or if you're really wanting him to have some higher level spell slots for upcasting, while still avoiding delaying Extra Attack, you could do Hunter Ranger 5/Cleric X, and just ignore the nature-y Ranger stuff. Swap out the standard Ranger abilities for the TCoE alternatives, so no Natural Explorer or Favored Enemy. Take something like Religion Expertise from your Canny ability. Any of your Hunter’s Prey combat options would fit with an experienced combatant like a Warrior Priest, as would Favored Foe/Hunter's Mark, so there's nothing explicitly nature-y or ranger-y there. Pick clerical Ranger spells like Cure Wounds and Lesser Restoration instead of druidic Ranger spells like Entangle and Animal Messenger. And you still get a Fighting Style. It's not quite as good an option overall as Fighter/Cleric, but you would have +2 levels of spell slots compared to a Fighter 5/Cleric X, along with a few extra 1st and 2nd level spells known.

Best of all, neither Fighter/Cleric or Ranger/Cleric are anywhere near as MAD as Paladin/Cleric!

I've been looking at some Swarmkeeper builds lately that look pretty good for forcing movement into hazards. With the upcasts, something like that with Ranger/ Cleric or Ranger/Druid looks pretty good to me. 5-6 levels of Fighter I'm less convinced about; you just lose too much casting for the trade off . Multi-attack vs. a cantrip that scales with total level is... ok, but taking levels 4, 5, and maybe 6 primarily for that seems pretty marginal.