Other options may include building The Great Library to catch up in tech (it can get built on Sid with pre-building sometimes if you're at say 250 shields, or better, in the box for the palace and the AIs have no other wonders in progress, you only need 150 shields to finish it, while the AIs only need 160 shields and maybe even with a golden age they have no city powerful enough to finish it due to irrigation or no one has a golden age... the human player moves first also). Selling tech immediately for gpt or gold from the AIs on the turn after learning some tech may be possible. Maybe using that gold to purchase libraries and univiersities. Then trying to learn least common AI techs first and trading/selling. I remember once almost growing broke in the late industrial age on a Deity Standard Spaceship game and selling off my last iron, but then getting something like Nationalism as my free tech and then having no similar trouble researching until launching.
Espionage is another possibility. I know some people have been confused about it before, but stealing technology via the diplomacy screen becomes possible once Writing is avaialable. It may or may not be more expensive than purchasing technology.
In a COTM competition a while back, on Sid, I. Larkin also used loans for gold at some point and then did maximum research with those lump sums, and managed to sell technology before going broke apparently. At least aggressive, the trade rate for gold is 1 human empire gold per turn for 18 gold.
Loans combined with trade route pillaging comes as where things can get even more powerful. For instance, let's say that France has 10 extra gpt that they would pay for gold. I'd loan out that 10 gpt for 180 gold. Then I come back to Japan and pay them maybe 30 gpt for silks and that 180 gold. Then cut the trade route. The 10 gpt still ends up in tact, but our empire has the gold back. Then on the next turn (or possibly the same turn if we have enough workers and pillagers) our empire has 10 more gpt to purchase hard goods from anyone.
There do exist two catches though:
1. If you export some luxury or resource to an AI and pillage your trade route to the capital, your reputation will get trashed by failing to supply the luxury or resource to an AI. So, if there's good reason to export some AI a luxury or resource, there's a consequence.
2. Loans can only take out gpt of what the AI can make. Suppose Gilgamesh has no gpt at the start of a turn. Then we inject gpt to Sumeria first by say trading them 20 gpt for silks, then taking out a bigger loan of gpt including at least some of that 20 gpt, followed by another deal like 50 gpt for wines and the gold back, followed by a trade route pillage is illegal.
Why?
"Giving the AI more gpt that you can afford or accepting more gpt than the AI can afford.
This is disallowed as it results in putting 'free' money into the international economy. While fixed for C3C, it is banned for all versions for the sake of consistency."
In that case, we would have accepted gpt from Sumeria which was more than they could afford at the end of the turn.
Also, Gandhi can declare war, because his traders wouldn't walk a few miles or so
without a road. That allows possible variants like winning a conquest game without every having declared war on anyone.
You can't trade for Lux+Tech unless you have Great Lighthouse if you are playing Continents/Archipelago.
And even with the Great Lighthouse on an 80% archipelago map there may still not exist a trade route possible until Magentism or Navigation becomes avaialble. But, I don't know. Do you need to learn Navigation or just the AIs?
Not having a luxury or resource avialable might come as overcomable there though. There's information in the screen with leaderheads about their existing trade deals. Like, let's say that Portugal has two wines spotted in their territory. It reads 'wines 0' in their potential trade deals on the trading table. They have some line suggesting that they are trading something with say The Dutch. Maybe there's another possibility also. So, then we might declare war on The Dutch. Then sign Portugal into war against The Dutch. Then whatever Portugal traded with The Dutch no longer applies. So maybe Portugal has wines for trading now. Or maybe they had some other deal going on, but the lines reduce the number of possibilities.
Or maybe The Dutch lie next to us and 20 turns of war seems out of the question for the moment. But 20 turns of war with Portugal is another matter. So, our empire declares on Portugal. Then we sign the Dutch against Portugal. What will happen to those wines if they were trading to The Dutch? Will they end up avialble in 20 turns?
Also, if an AI doesn't have any resource or luxury on the start of the turn, there may exist another possibility for getting them a luxury or resource. I think I learned it from looking at
Gyathaar's save.