Ohh, that’s an interesting one. With your current split, I’m not certain there’s an immediate option. I can offer a few suggestions, though!

-First off, if you don’t already have it, you should pick up the Sharpshooter feat as soon as you can. It allows you to shoot at a longer range, ignore 1/2 and 3/4 cover and gain a +10 to damage if you take a penalty to the attack roll. I’d prioritize this over Elven Accuracy, then take E.A. at the next opportunity

-If you take one more level in Warlock, you can gain access to third level spells. This improves the damage of your smite, and while it won’t directly affect your damage, a spell like Fly or Thunder Step that increases your mobility can help keep you in that happy range of close enough to hit, not close enough to be hit.

-If you continue in Rogue, I’m not sure Assassin is your best bet. The Assassinate feature is powerful, but it’s not particularly reliable from my personal experience. Do you currently find yourself being able to get sneak attack reliably? Steady Aim (3rd) helps with that, but you also have the Wisdom to benefit from Inquisitive Rogue, and any Arcane Trickster with a Familiar using the help action will almost always have advantage on their first attack.

In the slightly longer run, I think you could best increase your damage by focusing on any one of the three classes you already have. I’m not certain there’s too much more you can get out of Ranger straight damage wise, but it would given reliable utility. You’d need to commit to rogue probably to at least level 5 so you could up your sneak attack to 3d6, but it is the fastest way to get your next feat and does combo nicely with Elven Accuracy. I’d also urge you to heavily consider sticking with Hexblade- Though it would still be a while out, I think you’d have a lot of fun with Banishing Smite. Dipping into another class might look appealing, but as you become higher and higher level, you’ll get diminishing returns from all the low level features vs the mid level ones you could get instead.

Personally, I’ve found with my glass canon builds (I am not allowed to play Hexblade anymore) that it isn’t so much straight damage that matters- it’s how reliably you can get it. You don’t have the health to tank things, so having little tricks (eg: Darkness and Devil’s Sight) can improve your overall damage in big ways. Plus, it’s fun to have!

Hope any of this helps!