I want you to be my sensei. What does sorcerer do in that build besides give haste?
Speeds up spell slot progression, giving you enough slots to spam more higher level divine smites. Invisibility comes in very handy on an Assassin. Also (quicken spell) + (haste/ mirror image/ other buff) is highly useful on a frontline fighter.
Laughably, for defence it also gets Charisma to saves, access to the shield spell, mirror image and counterspell, heavy armor. I expect (in full plate) it to have an AC of 19 (plus 7 from haste and shield) fairly routinely, not including items.
The fact it also has expertise in two skills, cunning action, two fighting styles, battle master maneuvers, action surge, channel divinity, lay on hands, is immune to fear, can pick locks and disarm traps, several metamagic options, Second wind, and 2d6 sneak attack is really just gravy.
Oh... and 6th level slots (with 2nd level paladin spells known and 4th level Sorc spells known).
You could ditch Sorcerer for Bard (Lore) if you want the exact same slots (and smites), but a more limited spell list in exchange for expertise in two more skills (again), jack of all trades (including a +3 to initiative), song of rest, bardic inspiration (and cutting words) and proficiency in a further 4 skills of your choice (just so you can laugh at the Rogue 20 crying next to you). You can used your Lore Bard 6 ability to plunder 2 spells of your choice to make up for the less cool bard spell list.
In the latter option, you get to buff the party and sing about how awesome you are before turning invisible, using your bonus action to hide, slinking into a monsters room, and going all righteous murder on its ass.
Only weakness is ASI's. You only get three. You'll need them though as one is going on GWM (to trigger the bonus action attack on a crit, which is guaranteed thanks to assassinate), and Alert looks good to guarantee going first with an additional +5 to initiative and preventing any fool from surprising you.
You can dump dex (heavy armor and expertise in stealth, pass with out trace if needed), int needs only an 11, Con and wisdom 10-12's , with Strength 17 and Cha 15. First ASI is split between Cha and Str. Take Fighter at 1st, then Paladin for the full 6, then assassin for 3 (bringing your assasinate + smite online) then another of fighter for action surge, then sorcerer (or bard) for the rest. Plays like your standard Paladin tank from 1-6.
I advise doing your best to get a belt of Giant strength, and whatever item spams Cha to 19. A +3 greataxe and a cloak of invisibility rounds it out nicely. Grab a potion of gaseous form as backup to escape any forcecage shenanigans also.