D&D 5E Arcane Trickster into Warlock

[Gidonihah]

So my lvl 10 Arcane Trickster Rogue accidentally got herself pacted to a Great Old One and was given a 'free' level in Warlock. Now I'm trying to figure a way to pursue going 8-10 lvls in Warlock without completely wrecking my character.

Applicable Stats: 20 Dex, 18 Int, 16 Con, 10 Chr
Feats: Keen Mind, Observant, Firearm Proficiency
Expertise: Stealth, Perception, Investigation, Sleight of Hands

Notable Homerules:
1) I'm allowed to use Int as my Warlock casting stat.
2) Renaissance level firearms exist, and they use Pathfinder armor piercing (despite touch ac not actually existing in 5e) in exchange for taking a 'move' action to reload.
3) AC and to hit is mildly inflated (Saw someone with a +3 Shield and +3 Plate Armor, and +3 weapons are possible), probally a relic of the GM not quite being adjusted from 3.5.
4) Due to the lvling system I will always be a lvl higher than the other party members. In a fashion this means I can be unoptimized to the point of falling one lvl behind.
5) Custom spells/items can be requested, but is subject to GM balance.

Flavor: My character has a ninja accountant vibe going on.
Party: The party has little melee and a lot of spellcasters (Cleric, Sorcerer, Druid, Wizard), despite this ranged sneak attack is still pretty dependable.

Any ideas for how to optimize from here?

 

[transtemporal]

Yikes, maybe bladepact for the extra attack and darkness/devils sight? You could do the eldritch blast thing once it overtakes your sneak damage (since you won't get sneak attack dice on spells).

Warlock doesn't add to your arcane trickster multiclass casting levels like everything else does either which is a bit of a shame, but I guess you'd get 2 5th level Warlock slots at 10th.