Firstly, I'm not sure why nobody has responded to this thread sooner, but I presume you have since played on! I'll continue my blab on the basis that you haven't, or that a retrospective view of your 50 B.C. game might be of some value.
You've done very well in killing off Korea and picking up bags of land and two very good cities. You've taken a fairly hard-line position on the Farm Economy, and as you've noted, your economy is 'in the toilet'. You've been fortunate that Buddhism has spread to Zulu lands and Ottoman lands, and that has opened the options for more of a religious-based approach to the game.
There are loads of issues that you've got to address;
What to do about Montezuma?
A few ways to go on this one;
You can try to convert him to Buddhism - therefore you'll need to (a.) swap back to Organised Religion or (in my view preferably ...) build some Buddhist Monastaries, and (b.) Open Borders with him, which poses another risk in that he may start sending Settlers into your turf.
You can let him be, and possibly gamble on an attack on Pyong'yang, then arrange a dogpile on him.
You can press on with an attack.
Me? I'd try for the Buddhist conversion, but be quick about opening and closing off the borders.
Wide open spaces
You need to settle quickly. You don't have open borders with anyone, but Mehmed can easily begin settling on your eastern coast. Draw up a dot-map, and do what you can to cut off access to your continent down the eastern coast as a priority by settling a city that won't allow Galleys to get further down the coast - this fifth city will be costly, but I think worth it. Cities that can work commercial tiles, coastal, and close to the capital would probably be next in line.
Cottage vs. Farm Economy
You have recently swapped your civics, so must wait for a while, but you're getting very little value out of the Caste System. Thebes has secured one extra Specialist Scientist (two from The Great Library, two from the basic Library, and you've added one via the Caste System), while Pyong'yang is running a Specialist Scientist although I question its value when that city has growth options. I'd look at Slavery again - cheaper, and allows quick production of essential buildings and units. As an aside, you've got a 'stop growth' on Thebes, yet it can grow without issue.
Start working more commercial tiles and research Currency. You've only got four cities, and an extra internal trade route is not likely to change much for now, but you may be able to sell off some cheap technologies for gold, while working on getting up more Cottages (for the 'Hybrid' economy). Build Courthouses in your two Korean cities. As you settle, you're costs are only going to go higher - again, work commercial tiles, swap into a cheaper civic (i.e. Slavery), and otherwise do what you can to address the economic position, working your way also towards the Forbidden Palace. Note that shrine income from The Mahabodhi isn't going to be great right now (very few cities have Buddhism) so while you should look at it at some later point, I wouldn't make it a pressing priority.
Other Tribes
Aside from your looming issues with Montezuma, I'd consider gifting Meditation to Shaka and Memed - they might build Monastaries and start producing Buddhist Missionaries on your behalf! You'll probably get a positive diplomatic modifier out of them too for sharing your technology. Otherwise you can hold off for Currency and sell Meditation to them. Get rid of the Barbarian city if you can.
Angkor Wat
I'm not a big fan of this Wonder, but I do concede that you have a lot of Shrines to build, so running Specialist Priests isn't the worst move in the world. You're not Industrious and you don't currently have a Stone resource, so I'd question the move in light of all of the other issues you have on your hands.
When you share a religion with Montezuma, you're half-way to 'getting out of jail', but you can't ever afford to be too complacent. 
