Here is a stab at an assassin based off the 5e one.
1. I replaced auto-crit with 2x sneak attack damage in 3rd level feature. This weakens it, especially for MC dips.
2. I added a poison feature at level 9. 2d6 free poison damage, with a crit rider.
3. I dropped Imposter time from 3 hours to 9 minutes.
4. Death Strike now lets you spend an action to up your next Sneak Attack on the creature to 1d12s, and on a crit save-or-die. Outside of combat, it means that an Assassin who has seen their foe gets another damage boost 1st round. In combat, burning an action to double your SA damage is only good if you can't SA on a given turn; so it still has some utility. The crit save-or-die is mostly a ribbon; most foes this is OP on have legendary resists.
5. Infiltration is (prof bonus) identities. You get them for free when you get the feature, and you can make new ones later (up to your prof bonus). This both bounds it (no books of false identities) and makes it less of a "oh well, no downtime, no feature".
Bonus ProficienciesWhen you choose this archetype at 3rd level, you gain proficiency with the disguise kit and the poisoner's kit.
AssassinateStarting at 3rd level, you are at your deadliest when you get the drop on your enemies. You have advantage on attack rolls against any creature that hasn't taken a turn in the combat yet. In addition, you roll twice as many sneak attack dice on any creature that is surprised.
Infiltration ExpertiseStarting at 9th level, you can unfailingly create false identities for yourself. When you gain this feature, you have your proficiency bonus false identities you have managed to create. You can create a new one by spending seven days and 25 gp to establish the history, profession, and affiliations for an identity. You can't establish an identity that belongs to someone else. For example, you might acquire appropriate clothing, letters of introduction, and official- looking certification to establish yourself as a member of a trading house from a remote city so you can insinuate yourself into the company of other wealthy merchants. If you create more than your proficiency bonus false identities, you must lose track of one.
Thereafter, if you adopt the new identity as a disguise, other creatures believe you to be that person until given an obvious reason not to.
Plethora of PoisonStarting at 9th level you can produce a sufficient supply of poisons. You can prepare up to your proficiency bonus in slashing or piercing weapons with poison on the blade during a short rest that last until you next take a rest. When they deal slashing damage, they also deal an additional 2d6 poison damage; if you are not wielding the weapon, the poison wears off after it deals damage.
In addition, if you score a critical hit with a poisoned weapon, the creature taking the poison damage must also make a constitution saving throw (DC 8 + your proficiency bonus + your intelligence bonus) or become poisoned. While poisoned this way, they repeat the saving throw at the end of each of their turns, taking your sneak attack damage dice in poison damage on failure, and the poison wearing off on success.
ImpostorAt 13th level, you gain the ability to unerringly mimic another person's speech, writing, and behavior. You must spend at least three minutes studying each of these three components of the person's behavior, listening to speech, examining handwriting, and observing mannerisms.
Your ruse is indiscernible to the casual observer. If a wary creature suspects something is amiss, you have advantage on any Charisma (Deception) check you make to avoid detection.
Death StrikeStarting at 17th level, you become a master of instant death. As an action you can observe a creature you can see and study its weak spots. Once you have done this, the next time you deal sneak attack damage dice on the creature, your sneak attack damage dice become d12s. In addition, if this attack is a critical hit, it must make a Constitution saving throw (DC 8 + your Dexterity modifier + your proficiency bonus). On a failed save, the creature is reduced to 0 HP and killed.