It is really great that you completed Communism and The Kremlin. When did it get built? How many hammers and turns do you estimate it saved your end game?
Ok, 1st of all: The Kremlin got built in 780 AD, so about 40 turns, before the game ended. At that time of the game, I had an Army of 80 Grenadiers, 20 Cuirrassiers, and 50 Cannons. That army would have been able to conquer the Rest of the map without problems, anyhow I still remember, that because of the map being huge, troops did take extremely long to travel from one empire to the next, the reason, why I transfered from Grenadier / Cannon warfare to the way faster mounted units, which saved me about 20-25 turns (so about 5% of the total game length) .
The mounted units however had high loss rates, even though it were Cavalries in the end. Therefor, I come to the conclusion, that I made the right decisions, with deciding for Praets + Siege in the beginning, as otherwise, I would have needed even more production, which would have been impossible.
Remember: The 1 Movement Units + Siege are more

effective, than the mounted units, while the mounted units are faster. Proof for this lies in the fact, that I lost almost no Praets / Grenadiers / Whatever but had a loss-rate of almost half of all Cavalries (150+ losses here) .
Therefor, the question how many

the Kremlin saved, isn't the really important one, as it came too late. The game was clearly won at that point, would have been winable without the additional Cavalries, so the only effect of it was, that I could conquer the last 2 targets a little bit faster because of being able to mass up troops a little faster, so I'd say, it maybe saved about 5 turns (about 1% of the game lenght) .
Anyhow: Communism was a great factor, as it was almost impossible to pay the maintenance, especially with the non-FIN romans and without running almost no Specialists. In this game, most cities were set to work a mix out of Food and Hammers. Therefor, I always had a great Military, was #1 in production from shortly after the beginning, but never had a great Research. Still: Communism enabled lower maintenance, giving the opportunity to whip more units and work less cottages, it allowed to fasten up the Research so I could invade Lincoln before he got Rifling so 2 things, that really had an impact.
So the decisions were good, the errors I see from a today's perspective lied in other parts of the game, on which I will evaluate in the followup:
Can you detail your technology path a bit more? No doubt the city pillage wealth helped keep the economy running and maybe supported a higher research slider. Was research more commerce based or specialist based? Did you steal many technologies? What technolgies did you get in peace deals?
- I got 0 technologies by peace-deals. I don't know how the mechanic behind this exactly works, but I know that bribes on Marathon are cheaper, because the techs are more expensive. Because the techs being more expensive, while
being quite cheap, I guess, that it's almost impossible to get any techs in peace-deals, at least, I was not able to ever achieve this with these settings. (So Huge / Marathon isn't easier in every discipline! )
- City pillage was a part of the economy, but an extremely small one, as I kept the land from the AI as it was, and therefor didn't wanna cut myself into my own flesh by pillaging it. In addition: On Marathon settings, pillaging is not very attractive, as Cottages take longer to grow, and the scaling of the maintenance isn't 100% perfect from Fireaxis, which somehow gets equalled by the fact that one has more trading-partners on Huge Maps, the reason why the Victory Dates are almost the same for small to large maps. In short, it's easier to bulb and research on faster settings and smaller maps, because of techs being relatively cheaper, GPs being more powerful and maintenance being lower. This is made up by the larger trading opportunities on larger maps.
- I also got 0 technologies by tech-theft. As to my (limited) experience, tech-theft is something from the stage where Jails and Intelligence Agencies are available, or, in Specialist-Economies working Spy-Specialists unlocked by Courthouses. This game was won too early, and / or had the wrong economy for the mentioned opportunity.
The Economy worked like this:
In the beginning, I didn't need any Research at all past Oracle, as the oracled tech + the great trading opportunities on huge maps can keep someone in the game forever. Therefor, I specialized 5/8 cities to Farms / Mines, only 2/8 to Cottages and 1 of course was the GP Farm.
This is good, because it gives the empire an instant kick, as Mines work to their full potential from the beginning.
I then conquered Willem, and thankfully, the AI builds and grows lots of Cottages and it's relatively easy to conquer large, well developed cities from it, as AI whips way too little. Those cities I kept, and didn't re-specialize, I only built the HE in one of my Core-Hammer-Cities and re-designed the Food-rich-conquered Capital to be the 2nd Military City (Globe-City) .
Biggest Mistake in this time: Building too many AP-Temples, because thinking the round would take longer and that they would pay back.
Good thing: The conquered cities were perfect to have the necessary theaters to be whipped, that did just get available at the time with Drama.
2nd biggest Mistake: Aiming for Music (instead of Civil Service / Paper -> Education Route) and building the right Buildings, at the wrong time!
Now that one I have to explain: A common error many players make, is, saying: I 1st will build the buildings I need, I then will produce the troops, and even after that, I'll conquer the whole world.
The problems caused by this, are, that the Conquest on the 1st empire will be late, one will need more troops. In the end, the cities are overbuilt, and one doesn't know what to do with all the troops, as the opponents are way to weak, even on Deity.
Remember: Greatest advantage in military warfare ist speed! Expand fast, build Units as early as possible, conquer someone. THEN, develop some cities, but simultaniously enlarge your army, and directly conquer the next empire! Stay away from all rules, like "every Commerce-city must have a Library" (or even a University) , and don't use build-queues! Only decide for the next building you want to build, try to sting directly, attack with all force and as fast as possible. The earlier advantage is the more important one, as that one can and will multiply!
Hope this (Wall of text) was at least a little interesting to read, and if you have questions, feel free to ask.
Sera
P.S.: In my video, you'll see the difference from this,

approach, to a more sofisticated technological approach, and will also see, how much better my game has gotten in the meantime and how well the things I said in the last paragraph help.