Okay I think I have a working model for it:

Level 3

Iaijutsu — During your Attack Action you may Concentrate and “store” your Attacks, then unleash them as a single Bonus Action attack on the same turn or in response to being attacked before your next turn (this also costs your Reaction) adding 1d12 extra damage to this single attack per attack stored.

Your first attack after drawing your weapon also has a crit range of 19-20.

Level 7

You have Expertise in Insight

You gain advantage on Initiative once per short rest

Level 10

You may sheathe your katana after an attack without costing a bonus action

If you kill a character with an Iaijutsu strike you can apply the same attack roll to another within five feet, up to Dex bonus times.

Level 15

Action Surge grants you an additional bonus action as well.

Level 18

Sheathing your sword by spending your Reaction lets you reroll damage after a hit.