Hmm, I think I spent as much time looking at save/thinking/typing these up as I did actually playing my last round. Apologies if these are a little too detailed...
OK, you might be able to get Sushi founded, depending on whether we win the GP lottery. Madrid has just birthed a GP so this is our best chance to get a merchant out of there.
Tech:
Sci Method is part-researched so we can trade MilTrad for that, Constitution, and Chem immediately - I THINK that Liz will go back to Cautious after peace and be willing to trade, you may be able to broker Sci Meth to Roosevelt as well. You'll probably have to research Bio and Medicine yourself (unfortunately that Great Scientist will bulb Physics before Bio, maybe someone besides Wang will research Physics and we can trade in time to use the GS on Bio but seems unlikely), hard to tell if that's doable without making peace and waiting a turn for the WW to subside. Probably will be, you can build wealth/research almost everywhere and run tons of specialists in the non-legendary cities.
WFYABTA limits -
Hatty 10
Lizzy 15
Roosy 15
Sulei (in case we drop below Friendly) 10
Wang 10
Zara 10
Since Wang, Hatty are in WFYABTA and Zara is not, I'm guessing we're right on the edge with those him (and he's forgotten one more trade than the others have).
So trade with Zara first.
Looks like several AI's have Corp., IIRC only Friendly civs will trade it to you in BTS. Maybe by brokering SciMeth to Roosevelt you can get all three of Const, Chem, SciMeth without giving MilTrad to Sulie, however by waiting a turn to broker to Roosevelt there's a risk he will make his own MilTrad -SciMeth deal. Since we will research faster without Corporation (most cities are coastal, so Corp is going to be -1 trade routes in most cities) I would just make a deal with Sulei immediately for one of the other techs and trade him Bio or Medicine for Corp.
However, trading for Corp earlier would allow a sooner revolt to Representation to speed research -
losing the HR bonus with Sulei will probably drop him from Friendly which we need to trade Corporation.
Other Civics - when you are able to switch, Caste is obvious so Madrid can run max merchants. Merc for an extra merchant in Madrid is useful but FM would speed up research a ton, your call. Represenation once we've made the deal with Sulei. And of course Free Speech.
At the current save it looks like Pacifism unit cost vs. no upkeep is roughly even, but that will drop once we aren't in Vassalage. You should be able to get the GP in 20 turns w/o Pac even though the GL and Parth will be obsoleted. (In that case, OR for the christain missionaries. Looks like Hatty may spread us Conf once we aren't in Theocracy too). Madrid has food support for 6 merchants and some room for food depletion which will allow odds of about 2/3 for the Sushi founder.
Other things:
While we are still in slavery you may want to whip monasteries in the Christian cities to get a jump on the spread. However, you would have to hold off on the SciMethod trade so probably not worth it. I did a quick test in another game, even if you whip a monastery and then trade for SciMeth before the next turn, it disappears. So this probably isn't worth it, make the trades and just revolt to OR 5 turns in.
Seafood - there's a fish 2 SW of Phrygian that should be hooked up if we get Sushi (not in the BFC of any city though). Pharsalos's borders should pop during your set bringing in another clam.
You should probably be able to finish all of the Christian temples and some of those cathedrals. The conquered Stone will help a lot with this
. Hermitage in Toledo seems like obvious call as well. Globe is still at 2/3 completion in Granada, Toledo could use a Theatre also if you are able to get all of the other builds done.
Granada is also getting close to a GP, so run artists there to try and cash it out during your set. Be careful not to pop before Madrid's unless the extra cost won't push the Madrid GP past the end of your set (note that making Madrid get to 1000 GPP instead of 900 will also slightly increase merchant odds since we won't be able to run max merchants until 5 turns in). I think artist should bomb Toledo; assuming we stay in free speech until the end settling would get more culture but I don't think the other teams (dominators especially) will leave that civic in place. As for location of Sushi - I like Madrid since Wall Street cannot be built there, that way if the spacers spam it to all cities it won't speed up their research quite as much.
If you get to the Sushi techs with turns to spare, turn off research. The only thing that will help us are Broadcast Towers and I expect the other teams will get us most of the way there. Well, other culture corps could help but that's highly dependent on GP luck.
One other thing: the last two turnsets have had civics changes on the last turn. This is partially my fault, I changed on the 2nd-to-last on my turnset, for a reasonable cause (Caste for a while to squeeze hammers out of a couple of workshops @ Madrid, then slavery to whip that first Christain missionary and was going to whip ND but it got build by someone else) but the last two have seemed gratuitous. I recommend not continuing this streak and asking other players to try to do the same.