Well, there is lots and lots of room for improvement. Expansion for T66 is very slow and I think you should have at least 5 cities by now. You should not build warriors to fortify them in your cities! One warrior for

in each city, rest should be spread OUTSIDE of your borders to spawn bust. You should maybe have like six total now, rest is not of much use. These units you should never upgrade, no. It doesn't matter what type of unit you have in each city, they will keep the city as happy as warriors than as riflemen.
I've already torn apart your unit management and worker management... Now it's time for city management.

You should not slow build settlers/workers after you've invested into granary! That granary does NOTHING when you are stagnating. However, when you are fluctuating in size (growing, whipping, growing back) that granary cuts the

-bar in half. That is insanely powerful. So if you want a settler, you should grow the city to size 6, put one turn into a settler, whip. Build barracks if you have nothing better to build. Put failgold into Stonehenge. Now that you have writing, start putting

into a library.
Your 2nd and 3rd cities don't benefit from a monument. All your strong tiles are on the 1st ring, as they should be.

Put

into a library rather, it's a building that is very useful until the end of the game. Mines are actually not very good improvements and I build barely any. It's better to just grow to work a cottage (or even a lake). Oh btw you are not working your cottages in the capital...
Another thing I almost forgot, what about food shortages? Is that something I should be worried about this early? In the past I usually add up all the squares in the BFC, after border pop, and build farms if I have a shortage. Is there maybe a better approach to take instead? I've read this in the war academy not too long ago so I just went with it. That said, it seems like Osaka and Tokyo are gonna have shortages at some point? Unless I'm missing something.
Stop reading war academy. Most of the stuff there was written over 10 years ago when many of the mechanics of the game were barely understood. Read this:
https://forums.civfanatics.com/threads/beginner-help-the-basics.648469/. Despite the name it goes way beyond basics. It's more like "basics for approaching deity level".
As long as you are making some +5

per turn you should not build many farms, just go with cottages. If you are making less than +5

, then you are founding bad cities that have no strong

-resources. Improve food, cottage all green tiles, ignore plains, don't build many mines will take you through levels until immortal.