Okay, started watching the second part now, having a blast

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I have to say it was really weird that Theo hadn't built her unique building, I mean it is bonkers and she is guaranteed a religion. Anyways would have been nice for you to capture it

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"The trading is completely wonky in this mod" Nice description

. You get used to it, but yeah it makes min-maxing a pain, feels a bit more realistic than the fixed values of vanilla however.
Cities do not have indirect fire before you research gunpowder.
Another thing: If you annex a city directly when you capture it, the anarchy time is halved and you get the courthouse auto-invested. It's clearly not a good choice for all cities, but it would probably have been worth it for Constantinople.
Don't know if you notice it later on, but when you captured Constantinople, you got a Colosseum in the city, it is preventing you from building your unique building, the Arena. It is a pretty weird interaction and should probably be fixed, but at least you can solve it in non-puppets by selling the Colosseum.
Another thing I noticed is that you seem to avoid Circuses, and instead try to reduce unhappiness with castles and so on, while there is nothing wrong with that at all, the circus straight up gives you 1 local happiness in every city it is built in along with the reduction to boredom which may or may not be useful. It is also a requirement for the zoo (if i remember correctly) which has a scientist slot and decent yields.
Citadels can be built outside your border (like in the base game) but not inside another civs (including city-states) borders. There were some brilliant reasons for this that I can't remember, so I'll just make one up

. It's so you shouldn't get as screwed over by open border agreements.
Every specialist does not cost one happiness, I think it's actually one half or one third happiness per specialist (not a huge difference, but there it is)
Cities founded by Pioneers start with both a forge and a barracks, two buildings that are buffed by the Arena, meaning building an Arena in that city would provide you with +7 hammers which imho is enough to prioritize it over most other buildings (I usually also invest it)
The religious division/tension/whatever thing isn't number of follower of one religion vs number of followers of another religion. It is total number of followers in a city divided by total population (at least I think so), meaning even people who don't believe in anything causes division, just like on the internet. Temples also reduces this number a bit (not really that much, but temples are pretty decent any ways).
The big jump in price to invest in buildings have to do with number of cities, I'm not exactly sure why, but as soon as you have 8 or more cities the price just skyrockets, at that point you pretty much need to go into industry if you want to invest in buildings.