Assassin & Thief Switcheroo
Thieves and assassins may both be rogues, with a certain thematic overlap, but 5e could avoid it a bit better in my view. Does the Thief need to be the one that gets "supreme sneak" while Assassins are described as skulking in the shadows? And should both of them get a first-round boost as a capstone? I think not, so here's my attempt at switching some features around and filling resulting holes. There's a few other changes baked in here too, but the main focus is on 9 and 17.
Assassin
Level 3: Assassinate
You're adept at ambushing a target, granting you the following benefits.
Initiative. You add your Proficiency Bonus to Initiative rolls.
Surprising Strikes. During the first round of each combat, you have Advantage on attack rolls against any creature that hasn't taken a turn. If your Sneak Attack hits any target during that round, the target takes extra damage of the weapon's type equal to your Rogue level.
Level 3: Assassin's Tools
You gain a Disguise Kit and a Poisoner's Kit, and you have proficiency with them.
Level 9: Infiltration Expertise
You are expert at the following techniques that aid your infiltrations.
Masterful Mimicry. You can unerringly mimic another person's speech, handwriting, or both if you have spent at least 1 hour studying them.
Supreme Sneak. If you are Invisible because of the Hidden condition and it is your turn, you cannot be found even if you step into the open.
Level 13: Envenom Weapons
When you use the Poison option of your Cunning Strike, the target also takes 2d6 Poison damage whenever it fails the saving throw. This damage ignores Resistance to Poison damage.
Level 17: Deadly Ambusher
You can take two turns during the first round of any combat. You take your first turn at your normal Initiative and your second turn at your Initiative minus 10.
Level 11: Uncanny Cunning
Optional feature for Rogues, to alleviate their action economy.
On odd turns, you can use your Cunning Action as part of a different Bonus Action you take.
Thief
Level 3: Fast Hands
As a Bonus Action, you can do one of the following.
Sleight of Hand. Make a Dexterity (Sleight of Hand) check to pick a lock or disarm a trap with Thieves' Tools or to pick a pocket.
Use an Object. Take the Utilize action, or take the Magic action to use a magic item that requires that action.
Level 3: Second-Story Work
You've trained to get into especially hard-to-reach places, granting you these benefits.
Climber. You gain a Climb Speed equal to your Speed.
Jumper. You can determine your jump distance using your Dexterity rather than your Strength.
Level 9: Roving Aim
You've trained to be always on the move, away from your pursuers and towards your next mark. Your Speed isn't reduced to 0 by using Steady Aim.
Level 13: Use Magic Device
You've learned how to maximize use of magic items, granting you the following benefits.
Attunement. You can attune to up to four magic items at once.
Charges. Whenever you use a magic item property that expends charges, roll 1d6. On a roll of 6, you use the property without expending the charges.
Scrolls. You can use any Spell Scroll, using Intelligence as your spellcasting ability for the spell. If the spell is a cantrip or a level 1 spell, you can cast it reliably. If the scroll contains a higher-level spell, you must first succeed on an Intelligence (Arcana) check (DC 10 plus the spell's level). On a successful check, you cast the spell from the scroll. On a failed check, the scroll disintegrates.
Level 17: Thief's Reflexes
Your agility is honed to perfection. When you use Uncanny Dodge, you take 0 damage from the attack and can move half your Speed without provoking Opportunity Attacks.
In addition, you become more slippery when taking the Dash, Disengage and Hide actions.
Deft Dash. As part of the Dash action, you can stand up from Prone or make an Acrobatics check.
Disorienting Disengage. Enemies within 10 feet of you when you use the Disengage action cannot make Opportunity Attacks until the start of your next turn.
Hazy Hide. If you have the Invisible condition from hiding when you end your turn, you cannot be found until the start of your next turn.