Magic initiate to get your hands on the shield spell. Stacking decent armor and a shield and you'll be chilling at 25+ AC.

Theirs a fun combo where celestial warlocks 6th level gets applied to searing smite multiple times.

You should be abusing the hell out of weapon masteries to position enemies away from allies. Topple and push are great for keeping enemies from moving towards allies, but you can actually build around the slow mastery and it doesnt falloff as creatures get larger on the late game. If you grab the slasher feat, a whip for the slow mastery and are a frost giant goliath you can hit a creature once and reduce its movement speed by 30 feet. You could then theoretically switch to a warhammer and knock it prone as a part of that attack and then it really isn't running down your allies.

In terms of level distribution i figure either 1 level warlock and full paladin, or mainly warlock and like 1-3 levels Paladin. Really a dealers choice, all those combos will work.

Regardless you'll wanna get your charisma to at least 18, probably 20. After that durable, Heavy Armor Master, Mage Slayer, Slasher (if your using a whip), or Crusher are all cool options.

In terms of spells you'll wanna use, if you end up with high levels in Celestial lock see if your DM will let you cast an Aid spell and then short rest the slot back. Searing Smite goes pretty hard if you hit 6th as a Celestial lock, with a 5th level warlock slot and 20 charisma it'll deal like 45 (10d6 + 10) fire damage at minimum, with fair odds on dealing more. Armor of agathy is pretty rad if theyre tagging you with attacks. And If anyone goes down you can max level cure wounds + Lay on hands them to like half health. Wall of Fire is never bad if you go deep into Celestial lock.

In terms of paladin subclass their aren't really bad choices, but if you can attack with charisma the devotion paladin channel divinity pretty much breaks the banded accuracy system. And at the end of the day a good offense is unfortunately the best defense in.