A problem is that there is sort of a lovefest going on. I'm not sure about the AI-details here, when the various one can be bribed, but you may get in a two-front war when you declare on somebody. Everybody like Cathy, and everybody bar Isabella like Napoleon and Boudica. Based on my own game I'd recommend taking out Catherine at this point, as she is probably the strongest (though Napoleon is likely to have a big army because he is a unit spammer), but I'm not sure how this would work out, and if Boudica declares on you too, it could be tough. Would be good if you could get somebody to pleased and beg gold for a 10-turn treaty before declaring.
You are not taking advantage of resource trading opportunities. There is gold per turn to be had, and

and

to be gained, giving you more leeway for whipping and sending out garrison units. Haven't you been trading resources at all? I see none in the screen when opening the game. Over time this will improve relations with AIs. You can get +2 for this, and at least +1.
I gave wheat to Boudica for 3gpt, and iron to Napoleon for 3gpt (he has metal anyway). To Catherine: sheep for 2gpt, crab for corn, and deer for another 3gpt (for some reason she got another 3gpt after one of the deals, originally she only had 2gpt available).
This gives you an additional 11 gpt, about the same in health I guess, but unfortunately no more happiness. Maybe that will come in time. Always make sure you check the trading screens from time to time, to take advantage of it, as you will improve relations with the AIs over time, and get money and resources in the process. For example, trading away deer for corn is good, as you lose 1 health and gain 2, as you should have granaries everywhere anyway.
Unfortunately there is too much of a lovefest going on, and none are pleased with you, so it wasn't possible to get other AIs to stop trading with each other, but this can be a good tactic to slow down the tech pace (AIs will trade with each other, often), and reduce the lovefest. Even more so with bribed wars ofc, but that's not possible either right now.
I then gave away CoL for 70 and 110 gold to Boudica and Nappy. It bumped Boudica up to cautious. Do this if you wish. They might get CoL soon anyway given the lovefest going on.
By simple math and the demo screen you can actually figure out that Cathy has the biggest army after you. (297000 / 1.27 = 233000). Whoever you go for, you probably need some time to gather your units and heal them up. You have the GW though, so don't have to worry about barbs in the NW, so you can empty those cities. Though you may want some guys there anyway for when you can take out Shaka entirely. Think I'd keep my EP on Cathy instead of Nappy though, as she is the most powerful and is close. You'll get tech research on her much faster than on Nappy given previous EP allocation. And it would be good to know whether she is researching Longbows...
You have a big empire now, so perhaps it would be best to postpone the next war a bit (bar Shaka's imminent death, as he is no use to you as a vassal with only one city) and grow up the infra and economy instead. Grow your army, get out some catas, built courthouses and forges, and then be ready to really stomp some Russian or Celtic behinds with much stronger strength in the future. Another war soon might just become pyrrhic and expensive, and put you too far behind in the tech race. This is epic, which I have little/no experience with, but I think that means you have a bigger window for warfare, so it's probably fine to focus on your economy a bit, as the Praets will be strong for a long time anyway. I was still waging war with Praets post-1000AD, though obviously with Maces and such too. But behind Cannons anything works, and CR3 Praets are
very durable.
Unfortunate you can't currently trade for Poly, but think I'd forego Feudalism for the time being and go for Literature instead, then get up HE, NE and GLib. Those will really help your economy and warring abilities. Probably change capital to Istanbul too, though that should have happened already really as it's a much better spot than Rome, and more central to your empire, reducing maintenance costs. If you whip the courthouse there right now you can get 36

overflow into the Palace, which is something. Obviously a courthouse in a future capital isn't great, but the OF would be.
If the game goes on this long, Istanbul would be an excellent Wall Street city, with the holy shrine, which is already netting you a pretty nice income. Alternatively you could also 3-whip a quick library there, work two scientists, and hope you can get out a GS for an academy, then get out the Palace later. If you want to change capital of course. With all those floodplains, though, it would make a pretty good cottage spot once Buro is in. Food isn't great with just a cow feeding it, but it will be allright with the 6 cottaged FPs.
Otherwise, get out forges in the rest of your cities, and whip some courthouses too. Will help your economy in the recovery, and get you some more espionage.
I see you have a settler ready. Where did you plan on sending him? One option is to found a city 1N of the northern gold, and then get the sheep from Ankara. It won't grow huge, but you can work the gold, and the copper too if you wish.
Especially if the idea is to postpone the next war a bit, you can send a unit down to Russia to check out what she has. Just remember that you'll need a bunch of Catas/Trebs for when the AIs get Longbows, as those things are nasty even for CR Praets, especially the hilly type.
This is probably what I would do, but you can go what path you want here, and you have more experience with Epic than I do
Oh, the capital can be moved to Cumae too, as it's pretty well developed and has a potential 7 FPs to cottage. I'd prefer Istanbul though, as it has more hammers. You also have a ton of forests to chop, which will help to get out infra (like Palace and NE/HE/GLib) and a bigger army.
Ho-hum, just checked your workercount too, and you need more of them. Would be nice to capture some more guys in the war, and you'll probably get some from wiping out Shaka, but you need some workers asap really, as 6 for 12 cities is too little.