I thought of that, but the sentient two-handed weapon means I can't use either of those Warlock features.
I'm not sure what you're referring to there. You may have missed the bit at the end of Hex Warrior (XGtE p55) that says "If you later gain the Pact of the Blade feature, this benefit extends to every pact weapon you conjure with that feature, no matter the weapon’s type." and the bit (from the PHB 108) which says "You can transform one magic weapon into your pact weapon by performing a special ritual while you hold the weapon."
Summary, HexBlade + Pact of the Blade should work fine with your sentient scythe/polearm, you can even take the Polearm Master feat to make it better.
Here's a sample build for you if you like:
Variant Human (with DM permission), start with 15 Con & Cha, boosted to 16 with racials, and 14 Dex. Wear Scale mail and upgrade as you can afford it to Breastplate or Half Plate (depending on whether you want to stealth or not).
For invocations choose Devil's Sight at L2, Thirsting Blade and Eldritch Smite at L5 (swap whatever else you picked at L2), whatever you like at L7 & L9, and then Lifedrinker at L12.
For ASI's, start with Polearm Master at L1, bump Cha +2 at L4, then Great Weapon Master at L8 (combine with Shadow of Moil spell for advantage on attack rolls to counter the -5 to hit) and then bump Cha +2 again at L12.
This should let you do very well at the "polearm wielding human who can see in the dark" plus gives you heaps of room for flavour and out of combat stuff. Pick skills that complement the rest of your char concept (eg stealth, intimidation, deception, etc) and use your other invocations such as Ascendant Step, Mask of Many Faces, Misty Visions, One with the Shadows, Otherworldly Leap according to your character vision.
For bonus points, come up with an interesting story for your sentient weapon Patron with your DM, perhaps even tie it in to the sentient scythe you're currently wielding ? Perhaps the scythe is an aspect / avatar of your Patron from the Shadowfell (the scythe / death flavour would very much suit this) and depending on your relationship with your Patron, might either pass on messages & favour from your Patron, or perhaps always tempt you into Shadowfell type excesses, your call.
How does that sound ?