Ethiopia - 1 city, 1 great general(on the offensive)
"that is my list of civs that have a nice advantage, but not overpowered"
This is classic HelloGoodbye123 analysis.
Ethiopia's power in MP comes from infinite city spam via religion, the opposite of 1 city. How can I say this?

Ethiopia is a faith machine, as its special monument(stele) gets 2 faith right away and city spam has no penatly for religion.

With Ethiopia in MP you will get a religion if you prioritize it,

and you select a faith/happy/culture building belief right away. Padogas are great, but sometimes somebody beats you to them. Anyhow, you faith buy the padogas/cathedrals/mosques ASAP without even getting a second prophet. As other religions get enhanced you keep piling on the padogas. So each city will have 5+ faith (stele/shrine/padoga) and the padogas will keep you up with the policies and give you happiness to keep spamming. By the time indu age hits I am close to 100 faith per turn without even enhancing my religion yet! (I go for other faith builders as well) I typically faith buy 3 great scientists later in the game from this.
In a good game I can spam 15 cities, a crowded game less than 10. I try to place as many cities on the coast as possible, as going liberty/commerce, +3 hammers per coastal city with 10+ coastal cities is killer.

How do I get so many coastal cities?

In MP players won't spam cities on those islands that have land but not unique lux. I don't need lux, just land! Against good players I have expansion constraints on the main land, but those outer islands are mine for the taking.

The admiral from commerce means I am exploring the islands in medieval, far before anybody else!
The science buildings go up late, as happy buildings, food buildings and units in these spammed cities come first. I will not be the tech leader until after the cities have universities, and then with 20-30 univ specialsts working science, I take the tech lead. When the public schools are put in, the cities are size 10+ usually, and with 30-45 specialists working science combined with rationalism, 1000 beakers per turn is the norm (1660 beakers my best so far.) All of those specialists spit out a lot of great scientists.
All of the scientists are saved for a slingshot - unless somebod bulbs to rush you with a military tech, then u gotta waste a bulb to get a military tech also.

Cuz 1000 beakers per turn is sweet, the last 8 turns of beakers per bulb is quite nice. Hence I am consistantly getting tech wins in under 190 turns (quick speed) if nobody pressures me.