My first impression was...ouch! As you say, you’ve been turtling a bit....and now may not have been the best era in which to war against the leader. However, you may still be able to turn this to your advantage if you can do some of the following:
(a) Switch to slavery NOW...you need it IMHO to whip in some naval defences ASAP. Given that the chap in first has steel but you don’t, the best place to defeat him is at sea, not on land. Moreover, IMHO, your biggest edge at the mo is that a number of your cities have plenty of space under the happy cap...use it by more aggressive use of the whip and you can get a land and sea army together pretty rapidly.
(b) Tech military science and then nationalism
(c) IMHO, depending on how many turns your war ally will take, get him to tech steel and agree to trade it to you (or tech it yourself if you can’t agree on a trade for it before you implement this course of action)
(d) Trade techs with your war buddy to get hold of steel if he’s teched it
(e) Switch civics into nationhood
(f) Draft and whip an army. Coastal cities build / whip ships of the line and galleons (and maybe a forge for infra), inland cities build / whip / draft cannons and grens. Some of your food rich coastal cities can also draft land based units. Be VERY aggressive with the whip and draft. Yes, your economy will suffer, but, if you and your war buddy are both willing to keep trading techs to each other, this will not matter....just keep researching different techs and then trade them. As far as production goes, as mentioned in (a), the key at this stage will be amassing as large a navy as you can...it needs to be enough at this point to hold off the chap in first place...and then turn defence into attack.
(g) To help with inland production, build a couple of markets / grocers / banks instead of wealth in a few inland cities (perhaps rotate them one at a time whilst others are building units)...IMHO, you need to end the reliance of your economy on building wealth so the cities can build troops – and markets / banks will help with this transition. Running four merchants (instead of a bunch of prophets) in your NE city once you pop your prophet in 6 turns will also help here.
(h) The move to stop building wealth will hurt your economy initially (until markets / banks / a Buddhist shrine etc are in)...so look at dropping the slider to 70% for a while
(i) Talking of your NE city, look at redeploying a number of your workers to this site...your NE city (and a few others) is / are crying out to grow - but need a few tile improvements to make this worthwhile.
(j) Some of your cities are building courthouses and The Forbidden Palace...but don’t have granaries or a garrison (edit: including your NE city) whilst you're at war???????
(k) When you attack, both of you might consider working together to take Krong Chaktomuk and Mahendraparvata first, depending on how close you think he is to oil based units. By the looks of it, take those two cities, and the chap in first has no oil!
(l) Going forward, I’d suggest you and the chap in third then need to work together to tech flight, artillery and combustion. Of course, assuming you both work together to do this and take down the chap in first place, it’ll be interesting to see how you then both proceed to finish the game...it could be a great finish in store for you all!
Hope this helps.
