dnd 5e – When would it be most effective to multiclass into warlock as a 3rd level swashbuckler rogue?

I am currently playing a level three Satyr Swashbuckler Rogue with the Sailor (Pirate) Background. I want to multiclass into Warlock, but I still want to be at least okay in combat with my mostly min-maxxed party. My current thought is that it seems reasonable to multiclass at level 5 or 6, and I’m considering both the Fiend or the Genie. We are planning to go to level 14. My stats are 8 16 14 10 10 16, and I primarily use a rapier.

I know that a Rogue/Warlock multiclass is not very good but I really like the flavor and want to make it work. I’ve been looking at maybe booming blade and eldritch blast for the cantrips and going to second level for invocations like Mask of Many Faces to help with non-combat encounters.

I have all sources available with the exception of unearthed arcana and Plane Shift.

I want to stay with rogue primarily, but I’m open to alternatives.

My party is a High Elf Illusion Wizard, a Variant Human Bear Totem Barbarian with the Sentinel feat, a Base Dragonborn Conquest Paladin, and a Lotusden Halfling Beastmaster Ranger (from Tasha’s).

If I need to include any other information please let me know, this is my first time asking a question. Thank you!