More on the upkeep cost.
I've a saved game SVN 652, that I'm currently playing. I'm playing at Immortal and Eternity speed.
Unit upkeep is THROUGH THE ROOF literally. Time suddenly jumps ahead 700 Yrs as well. The save was previously at 1149 AD now its at 472 AD.
I tried dropping down difficulty to Noble level, using flexible difficulty, and played for a number of turns till all I had left was 1 defender per city, all others had been deleted due to maintenance costing, I was in a golden age, and losing >450 gold per turn still.
2 extra screen shots are from Settler level, the City maintenance is still the same at 343

, but the unit cost does decrease markedly, which is to be assumed, reading the Handicap file settings.
Both the unit maintenance handicaps (over 350

on noble, 655 on immortal, 59 on settler) and the civic and city (343

) maintenance are grossly over estimated for this speed.
It may be balance for quick, but at this speed, the gold generated doesn't scale to the game speed played.
So a quick off the top of my head calculation, Quick = 1/12 of Eternity
12* costs, doesn't equate to a 12* gold generation for speed. The same level of gold is generated per turn, but the costs have gone up, over by a factor of 1200%.
Before, each new unit, would add 30 of so gold cost to my expenses, so I was balanced on the need for defence and cost.
It's not affecting the only AI I can see at the moment, as Huyana Capac has 12 Pop, a HUGE stack of troops in city, 2

on population and is running 95% science slider in the green. (Open borders, were you can see into opposing capitals.)
Previously I was playing at 40% science slider, barely in the green with 7 cities.
Apply SVN 652, and BANG I'm now
-1150 in the RED I now have the above mentioned city maintenance costing and I'm in a golden age in 2 turns, changing civics did very little, if not made it worse in certain civics.
I know balancing will take time, I do appreciate your time and efforts, I just want to highlight an error in assumption you've made.
I criticise, not because its easier, which it is, but its so much more fun the coping it.
P.S. I'm playing with raging barbarians, stacks of 120+ turn up every 20 turns of so to assail my border cities, I've disabled the max units per turn,0, as it was mentioned the AI's couldn't cope and find I have to use stack attack, which works fine, till the barbs take a city of mine, and it will get stuck, or I didn't wait long enough to allow it to process. No aggressive AI, as its been mentioned it dumbs them down too much.
The Bug errors I reported previously, it was an incorrect copying of my User Setting folder, its all fine now, sorry.