Earliest I got him was turn 130 with Mykara and Cultists.

What I did was keep workers at science 24/7 after half of Era I. Nothing else. Boroughs, production, and food only help science if I get them without dedicating workers. The only time I moved workers was to finish era III dust buildings and to get more dust to buy adamantine from the market.

I beat hard ai.

SO, obviously since I'm a noob, what can I do to rush Fotios even quicker with one of the one-city factions?

Abuse the hell out of tech trading and stockpiles (especially as cultist, but in vanilla other factions can just buy them from the market as the AI wont purchase them). Era3 should be comfortably reachable around turn 40 - 50 (on normal speed) if you do it well. Fotios is only a single tech and stockpile away by that point. 

If you ever want a more challenging AI, you might be inclined to try out the community patch though ;) (https://www.games2gether.com/amplitude-studios/endless-legend/forums/15-modding/threads/30618-endless-legend-community-patch )

You may also notice that the EL forums are a bit inactive at times. If you dont get answers here, you should join the official Amplitude discord, the EL channel there is a lot more active: https://discord.gg/yZYGZQ

I didn't know such a thing existed. Could you elaborate on how to abuse tech trading and stockpiles?

Turn 40-50 Era III seems like a dream come true.

edit: How do you get enough Dust to afford market place that early? I couldn't even assimilate a minor faction as a cultist without going hugely into negative dust.

The cultist have a relatively easy time getting a bit of dust early by getting the mercenary market tech soonish and then just selling all the free units they get from converted villages. You just put all your pops on influence, do as many quests as possible asap, convert villages and sell the units (Obviously you still tech the standard era1 FISDI buildings). 

Once you hit era2 and have the tech that gives you science stockpiles on razing, you can think about doing a surprise attack on some enemy that isnt an immediate thread for you. Just raze one or two of his cvities, get dem juicy stockpiles, this will massively speed up your city development and tech progress. Remember to keep some stockpiles for the moment you hit era3, because they get upgraded once you get the "unskilled labor" tech from that era. You can also use all the pearls you get early on to build up pearl stockpiles for "unskilled labor".

As for tech trading: just get diplomats manse in era 2 and make peace with some empires. Trade techs with them. Dont be afraid to make unfair deals in the AI's favor (within reasonable boundaries). Even if you hand over two techs for one tech each time, if you do that with 3 different empires, all these empires gained two techs while you gained 3. You can also trade luxuries/strategics you dont need against techs or dust. Since a diplomatic heavy/trade heavy play style requires some influence, you probably want to get "Glory of the empire" asap.

Note: you dont necessarily need to do all these things to be successfull. These are just some options on how you can snowball faster.