I can only talk SP standard speed. Which is likely no use to you but here is some of that.
maintaining decent science and gold output
Nope, you do not need good science output, just good culture initially. Did you read the post in another thread on phases, that clarifies a lot of the feudalism strategy.
Gold initially is more about selling unless you are pushing for medieval fairies inspiration.
The expansion phase is about getting settlers out, you do not finish districts you do not need to for eureka/inspirations. It is the main mistake people make with it. At Feudalism you get 5+ charge builders and it is around then you do the big majority of your chopping including chopping in districts which slingshots you past the AI because you have more cities.
Because expansion is about war as well as settle you can be on 20+ cities by T100, I think my most was 28.
Probably a mix - many people reroll, I tend to play at least 20 turns regardless but yes, poor starts like tundra you have no chance of what is quoted.
Being attacked by barbs is a big setback, dealing with barbs is a key thing in the first 10-15 turns. Not sure what you know so apologies for what you do.
A scout needs to see your city to get an exclamation mark, it normally then sprints off to find the nearest barb camp... I think it knows it’s source camp but unsure.
Sitting a city on top of a hill means a scout can see it from 3 tiles away
Sending your warrior out in a straight line in increasing your chance of a barb attack as opposed to circling your city, in particular looking for horses (early AH helps)
Getting a scout rather than a slinger first really makes a difference, send it out the other side of your city from your warrior... it has a ZOC and scouts do not like to move through ZOC. Slingers naturally do not have one.
Early barb horses are pussies, move 3 strength 20, even a fortified scout does OK versus them.
Emperor barbs spawn every 2 turns, immortal are every one, much more vital on immortal to stop barbs.
Naturally barb / camps give both gold and era (and inspiration + eureka)