Ehh... that's kind of deceptive here. In the first place, Colossus Slayer applies whenever the creature is below max hp. So, if ANYONE has damaged the critter in question, then it would be on the first strike; and you DO need at least one turn to apply the Mark without that extra attack.
The creatue will be below max hit points after the first attack, so it applies on the first attack you land after that on your turn.
You then get to use Riposte for an attack on its turn, again applying the 1d8 for colossus slayer, and 1d6 from hunters mark, plus the superiority dice, plus the weapon damage dice, plus your ability score modifier. (1d10+2d8+1d6+5).
Then you get to make 5 attacks, on your turn, each one at 1d8+1d6+5, with one of those 5 attacks getting an extra 1d8 from colossus slayer again.
Your standard 'kill it now' single target strike is (hunters mark) action surge 6 attacks (7d8+6d6+42) [Draw second sword] on turn one. On its turn, riposte when it misses you and deal another (1d10+2d8+1d6+5) damage to it. Then take your turn and attack it 5 more times, dealing another (6d8+5d6+30) damage.
Youre down an action surge, a 1st level spell slot, and a single Sup dice. Feel free to spam more to make it frightened or disarm it or knock it prone (adding 1d10 damage each do as well).
Probably land a crit in there somewhere as well.
Single target monsters die very fast. Unlike your GWM buddy, you hit at full proficiency plus Dex (no -5), so you're landing every attack
And yeah; youre wasting a bonus action laying on Hex every other round (you lose two off hand attacks, but gain an extra 3-6d6 damage from Hex, so its largely a wash when you do it, and then its one dead monster the following round. Meanwhile the DM is pulling his hair out dialing encounters up to quintuple deadly to challenge you.
If you really MUST buff TWF, I suggest allowing the Dual wielder to make an extra attack of opportunity per round (with the off hand weapon). That stops it from being OP in corneder cases, and makes dual wielders stickier.
I make TWF allowable with a single non light weapon (only off hand is light) but it draws disadvantage on the off hand attack unless both weapons are light.
Dual Wielder does what it does plus removes disadvantage on the off hand.
Bear in mind TWF and Ranger (Multiattack defence) also pairs pretty well with Defensive Duelist (as long as one of the weapons are finesse).
You parry the first attack with Defensive Duelist, and then the remaining attacks get -4 to hit you due to Multiattack defense kicking in.
Its a corner case fighting style that works OK for Rangers and Rogues (swashbucklers in particular). Its an option for Bards to up DPR in melee (seeing as they're not proficient in heavy weapons or for lore bards, shields). Corner cases.
Historically fighting with two weapons didnt happen all that often in any event becuase it was rubbish. Long pointy spear and shield was the way to go unless you were packing a specialist weapon of some kind. If you're wiedling a single handed weapon, then use a shield. They're better in every single way.
