Well, I tried a base Terra map instead of PerfectWorld6 and won...but I had a jungly start with lots of hills anyway so it might just have been a learn-to-play issue after all
This time I didn’t bother trying to get the eureka for Archery or spend ages looking for barb camps - actually I got almost no early goody huts and got scouted by a barb camp on turn 2. Neighbours were Canada, Ottomans and Mali, so denounced Wilf immediately as his land was closer and looked more open. My start location had great gold so I went war cart x3 -> buy builder, build settler -> forward settled Canada, slotted Agoge and built 4 slingers while researching Archery then immediately upgraded them, then carts in both cities constantly for a while as I got the cavalry card slotted.
By the time I was getting the last slinger out Canada had declared a formal war on me and it took til something like turn 40 to kill all his units in the jungle between us, so I was considering restarting. But he had a lot of farmland so decided to just push like hell, try to siege two cities at a time, only stop units to heal if they were practically dead and see what happened.
Well Toronto ended up copping a thousand-year flood that destroyed all its farmland right as I was about to heal, but fortunately also left the city nearly defenceless. Eliminated Canada (who somehow had 6-7 cities already) entirely by turn 74 with a bit of help from loyalty pressure splitting one of them off.
I was in two minds about whether to sit back and snowball or try to push Mali as well. Having never managed to wipe a neighbour on Deity I got a bit cowardly and decided to make alliances and snowball - considered a second later rush with Knights and crossbows but just didn’t have the gold to upgrade properly. Eventually won the science victory pretty comfortably despite a lot of pretty poor midgame play - switched my brain on late in the game and started using my cards more effectively plus suzeraining Geneva and other useful city states. Did have to go on a spy rampage to wreck the two stronger space competitors which worked well.
Can definitely see the appeal of an early rush with Sumeria now though I think I’d just restart if I got a start loc without great gold or production. If you can completely wipe out even one neighbour with good land it gives you a massive boost and the Ziggurats keep you from getting too behind early on. I feel like my main problem was wasting too much time trying to be ‘optimal’ with eurekas and timed promotions when you really have to focus on rushing as fast as possible in the first 50-80 turns. It’s also immensely helpful if you can siege cities immediately to deny them free hits on you while healing up. That’s one area I think PerfectWorld6 DOES make harder as it tends to create a lot of natural defences and less open cities.
edit: one other thing that helped is I planned out v carefully how to get the points for normal eras in medieval and renaissance including holding back on exploration etc - I’ve sometimes had issues with not being able to find era score during a golden age and early to mid dark ages are absolutely crippling on Deity. The +5 from eliminating a civ is massively helpful for this during a golden Classical