Theoretically, what's a better ghost move set for Gengar?

Probably Lick/Shadow Ball

There’s also unreleased Ghost type moves (I think?) like Night Shade

I just checked it in GoBattleSim against PC/FB zam. Lick has ~5% dps advantage over SC.

By numbers purely, Lick nudges out Shadow Claw. It trades 2.86 points of DPS for 3.43 points of EPS, theoretically letting it use Shadow Ball more frequently. This gives Gengar higher DPS and TDO on paper.

In simulations against C/FS Alakazam it's actually less reliable than expected. I think this is because rapid-firing Lick builds Alakazam's energy faster, causing it to fire off Future Sight early instead of a 4th Confusion. Overall they're probably interchangeable.

I just read a discussion about this in discord, it seems that it varies by matchup but mostly SC seemed to perform better. Takes a lot of simulations to find the exact truth.

Regardless of which one is better, Lick would be so much better than Hex or Sucker Punch that the trade value of SC Gengars would tank hard since they wouldn't be so special anymore from a performance standpoint.