Alright, followed your advice and did much better. Knocked out Cultural CS to left of my capital, then got a free policy (chose 1.5x military experience), then declared on Xia and took it, but 3 CSes declared permanent war on me for that (including Korea).
Then I crushed south China, and got 4 cities (!!) with no losses, but was forced to make peace after Darius, Gandhi and Ram declared war on me (lol). I needed the resources she was offering to keep my happiness up. Previously the unhappiness was around 12, and my troops took a -33% combat strength hit for that, so I decided to make peace to remedy that, leaving Wu's coastal cities and her leftmost city in her possession. I crushed Northern China, but still had unhappiness problems. I killed off Korea's pikemen and crossbows who were near the border of my leftmost Chinese city, but couldn't take Seoul thanks to the unhappiness. My Keshiks were doing 1-3 damage to Seoul, rather than the 5+ I was used to when I smacked south China early.
Persia eventually took my left cultural CS and then made peace with me after I killed about half his troops using hit-and-run keshiks. Still wasn't fast enough though! The main issue seems to be happiness, and the sheer wall of troops Persia puts up. Any more tips you can provide? The load game exploit for Deity wins no longer works