I'm seeing the same problem here that I've been seeing elsewhere on this forum. That is, people essentially saying "Your start is fine, you just have to play in this super optimised way, and if you don't do it this way, you deserve to lose". If I wanted just one optimal way to play, I'd play pong. The point of Civ is to play in your own style. Even on the hardest difficulty, there should be room for different styles. Sure, some playstyles like OCC should be a virtual death sentence, but not all bar one or two.
This is something that I'd like the Devs to look into for Civ VII, if it's too late for Civ VI. I don't want to have to choose between having agency in playstyle at the beginning and having resistance to military from the middle onwards.
That's unfortunately how it is in civ 6 deity.
I personally don't like it myself either, but if you intend to beat deity that's pretty much how you need to go about things.
The reason is that the AI quickly snowballs out of control in the classical era due to their inherent bonuses, where you have two options to pursue:
Either you take them out before the snowball really starts rolling (you rush him as efficiently as possible, which limits the consistently viable builds), or you set up for the long game where you plan to overtake them later on (you need plenty of room to settle and get infrastructure up asap, which also somewhat limits the amount of consistently viable builds).
As for the peaceful option you usually have a lot more leeway (even on deity), depending on the terrain, victory condition and your neighbours.
For the aggressive option, you really can't be flexible unfortunately.
The reason is that you
need to either kill him (or be able to hold at least 1 city or more in a peace deal) in order to set yourself up for later on.
The main reason for this inflexibility in regards to aggressive options, is that city defenses (walls and automatic combat strength increase upon fielding higher tech units) in this game are way too overtuned at the moment, which severely limits your window of opportunity in time for said aggression.
The AI nearly always builds walls early now, and is generally capable of fielding their first horseman/swordsman pretty early, and once that happens you are
not taking out those cities anymore with your warriors and archers.
Which can of course be a major issue, since if you are rather locked in in regards to settling space, you might find yourself locked down to just a couple of handful of cities, at which point it is often game over.
And in the meantime, the AI will just snowball further out of control while you lag severely behind.
Again, I really don't like it either, but that's unfortunately how it is due to city defenses these days.
Personally I think walls being overtuned is the biggest issue atm for the replay value of higher difficulty play in civ 6, as I can somewhat manage to take down a 35 combat strength capital with warriors. With walls though, that's just a death march. (And no, you can't just get a battering ram to "fix" that, since by the time you get it your units are even more obsolete and they usually have crossbowmen fielded)