Building a Spy/Assassin https://www.reddit.com/r/3d6/comments/zr76mz/building_a_spy_assassin_for_a_secret_military/

Go full mastermind or a 14/6 with glamour/eloquence, if you want an infiltrator build i would recommend a 13/7 phantasm/shadow monk.

I have to redirect you to the Whispers Bard:

Full caster, Psychic blades stays close to sneak attack, and at LVL 6 you can become someone that you killed (and use their surface thoughts) to infiltrate.

You can grab disguise self as a Bard spell, with expertise at LVL 3 as well, so you would really just need Crossbow Expert to get the most out of your damage add (jack of all trades on initiative will cover some of alert).

So I would stay mark of shadow elf but go Whispers Bard.

8, 14+2, 14, 8, 12, 15+1

With Crossbow Expert and cloak of elvenkind.

That being said, if you really want to go with rogue then your going to have to do a dip if you want to grab Eldritch Adept for Mask of Many Faces (you need the spellcasting or pact magic feature) so I would go with a shadow elf again with the same stats. Take Alert as your feat and hat of disguise as your item. You can dual daggers until you pick up Xbow Xpert.