First of all don't take to personally, one thing I am not know for being good at any gaming forums I have account on is manners. Oh well.
Two - I actually strongly disagreed lot of Sisiutl did in his games. Probably would disagree even more now. Yet I was somehow getting behind his thought process.
I am of course still only on T30 which is ridiculously early in the game. Its in that sense far too early to be developing a long term strategy, I need to explore the map and get established.
Really, I don't think so.
Lots of people at some time fall into trap letting small decisions slowly and gradually become defining ones. Like I need tech X now, I settle Y spot now... And at 1AD you can't do war or are short on trading partners or whatever, which removes lots of healthy options.
Some people would probably disagree but playing from turn zero with sorta plan in head moved me 1 difficulty up almost immediately.
Let's take boring me for example:
My turn zero plan is getting hold of my home landmass... since that basiically gurantees any win condition.
Naturally that would change a bit depending on situation -
can I get to all AI voia land
can I get enough land for domination on my continent
are there any obstacles - like 10+city having religion sharing Boudica next to me

and so on...
Short term goal on turn zero is scouting as anything else depends on it. Well, again having info to make your decisions is often overlooked aspect.
As game progresses I look at establish fare base to killing AI to get unhealthy share of land

. And by fare base i mean at least 4-5 really good cities. Lots of people aim for like 6, but all best national wonders - HE, Ne, maoi do not have city number requirement. Not like I would decline extra space to expand if available, but 4 cities usually gets the job done.
Now that is war orientated mentality, if you need good advice on peaceful life and build up, you will need ask someone else.
Growing to pop 5-6 before spitting out settlers and workers is certainly the standard way to play.
I did not suggested that, but how many turns you were into settler on founding copper? 3?
I would have canceled settler, grown up and resumed it. Likely gets settler turn or two later but you end with extra 6 yield tile and pop point.
Given difficulty Brenus is another 10-15ish turns [at very best for him] away from building his own settler, plus some turns adding escort. [that is from your last save point.]
Chopping instead of working 5-6 yield tiles in capitol is super rare to be good strategy.
Why was I going for an early settler at pop 2? To block Brennus. A close neighbour getting too well established is a good way of blocking my own development imho. One option is to rush a neighbour, another option is to try to keep them small enough not to become a problem later in the game.
Well, that is mentality thing again.
Rush on difficulty with AI having archery from start has rather high opportunity cost.
Personally I would rush if:
A)I end being blocked from expansion [overall I don't give damn of him expanding, dead guys are not problem later in game],
B) that is guy I can not live safely through buildup stage [Monty, Genghis and Co],
C) having quechas or being Prince or bellow - killing one AI has usually enough merits to do so in this case.
D) maybe if oponent rome and looks like getting hold of way too good land - Praets make classic age war very meh, Julius is not healthy neighbour
The thought of "AI less than ten tiles! Kill! Kill!" is usually not very productive.
From my warmongerish mentality I do not see why I would want to contain Brennus in small as possible area, beat him to key resource bubbles - sure, brick him to two city empire - not so much.
From my point of view being busy with blocking Brenus and leaving Gil extra share of land is unhealthy too - promoted vultures+protective longbows do rather well against cats and maces. I'd rather have celts at five cities and sumeria at five too, than Brenus at two and sumeria at 7 for example, even if i get extra city myself in later case.
Not sure where this does stand from builder viewpoint although Gilgamesh is lot of worse neighbour than Brenus. On top of all he has half a chance to actually pay for extra cities via zigurats.
So being me my short term points would be:
a) getting good second city oriented on production [1NE of rice], likely leaving southern part of FP fro brenus.
b)getting land scouted - locate Gil, locate good sites for cities 3-4-5-6 - pick these preferably towards Gil.
c)more mid term - tech to maces assuming more AI are founded.
And I suggest you take me few notches easier. It is not like I am looking for a bat to check in practice if pigs can fly or something.