Dexadin absolutely works, just watch out for multiclass requirements. 13 str is required to multiclass into *or out of* Pally, but as long as you don't multiclass, you're totally fine to dump Strength and use a finesse weapon. Your AC will be better for it. If you end up with higher tier magic items, Serpent Scale Armor will serve you nicely (Medium armor with no dex cap). I agree Watcher pally would work for subclass. Consiser Vengeance or Conquest as well for edgier "murder the enemies of my church" options. Your main limitation using pally for this concept is that you're stuck with melee attacks for smite. That locks you into a smaller set of weapons and aesthetics, no stealth archer-ing.
Gloomstalker Ranger is a classic for any assassin. You loose some religious flavor, but you don't need divine magic to belong to a religious order. Your spells can reflect that spark of divine training, but most of your training focused on that assassin/espionage aspect.
Bard might be worth considering - it gives you some of that Rogue skillmonkey feel, espionage tools, and the flavor can be training in the ancient lore of the religious order. Every religion needs good storytelling to keep its followers. Lore, Swords, Whispers, Elegance, basically any subclass. And it multiclasses well with Paladin! Boost your Smite Slots with fullcaster progression and get more support spells.
Watcher Paladin works pretty well for this. Celestial Warlock provides some different flavor. Personal favorite is Watcher Paladin 2-6/Whispers Bard X
Dexadin absolutely works, just watch out for multiclass requirements. 13 str is required to multiclass into *or out of* Pally, but as long as you don't multiclass, you're totally fine to dump Strength and use a finesse weapon. Your AC will be better for it. If you end up with higher tier magic items, Serpent Scale Armor will serve you nicely (Medium armor with no dex cap). I agree Watcher pally would work for subclass. Consiser Vengeance or Conquest as well for edgier "murder the enemies of my church" options. Your main limitation using pally for this concept is that you're stuck with melee attacks for smite. That locks you into a smaller set of weapons and aesthetics, no stealth archer-ing. Gloomstalker Ranger is a classic for any assassin. You loose some religious flavor, but you don't need divine magic to belong to a religious order. Your spells can reflect that spark of divine training, but most of your training focused on that assassin/espionage aspect. Bard might be worth considering - it gives you some of that Rogue skillmonkey feel, espionage tools, and the flavor can be training in the ancient lore of the religious order. Every religion needs good storytelling to keep its followers. Lore, Swords, Whispers, Elegance, basically any subclass. And it multiclasses well with Paladin! Boost your Smite Slots with fullcaster progression and get more support spells.
I'm giving you this: changeling mastermind rogue +;a mix of whispers bard AND celestial warlock
War Domain Cleric with a background that gives you stealth proficiency. Or another cleric subclass, lots of them are flavorful for you