I think to make war, you basically have to be able to affort it. And the way to affort it, is to have healthy cities and some happiness buffer.
Cities in bad location, just for sake of securing some area, without any good combination of food+production, to secure good initial growth and buildings, kinda holding you back
since they are usually behind in terms of building and bring "net negative" in terms of happiness, so cities you make should always have some healty starts including some effective
2-3 food and production tiles.
Also the real happiness often comes not from buildings and tiles, but flat sources of happiness, that is why i very often prioritize Stoupas in terms of religious buildings, since, the unhappiness
of a city cannot exceed number of citizen regardless of needs met (except urbanization), so +1 happiness basically is very hard to compensate with the right amout of prodyction/gold/science/culture.
Also very important is i think to keep track which cities will be unhappy fastest, and decide which are your main cities, and click prevent growth on cities that are not strategically important.
And for me strategically most important are usually a capitol, and first cities in really good locations and many improved resources.
Also, for example, if you just build Circus, you can growth city by 1 without any happiness complications.
To judge value of cities i kinda look on every working tile, and try to judge the value it brings couting the values as gold=1, food=1, science=2, production=2, culture=2.5 or 3.
If you dont have some unique tiles, with some resource or good impriovement, simple +2 food on unimproved grassland, then maybe its worth to just prevent it from growth and use scientist,
until you have some better value. Like, using above values scientist is 3 sicence -2food, so the value is 4 instead 2 for 2 food.
Also, important is to not make too big cities that are your main source of unit production, since production penalty is not worth it.
Kinda just thought about it, but maybe some cheet would be to make a settler in a city to decrease its size, but then again usually at the point you realise you need
it, it usually have penatly which make it not very effective.
Dont have exact math, but if needs are based on global median, then you kinda basing on your neigbour unique abilities and buildings judge, which resources you will be usually behind.
Like if there is china, they usually have good amounts of science, so you should prioritize those buildings, since it is obvous you will lack them more then other very soon.
You can also decrease gold needs in a city by sending some gold caravan from it.
Usually after taking 1-2 max 3 cities, you almost always need some period of stabilisation. It is good idea to save one caravan for each anexed city and immediatelly send production to such city
and build courhouse asap, so the city start contribute with its happiness.
Also, if you can secure some random great people, you coud use great musician, since now beside bonus tourism it gives +1 happiness in all cities..
Funny thing, there is a quest after building baths, that you can choose +1 happiness in all cities and +3 cost to baths, but at the same time Great musician tour will give u the same but without the
bath costs
Also, as soon as you research infrastructure project, you can try bo build it asap, before it gets +100 cost next era, besize one happiness, it resduce number of cities penalty and -10% all need, it kinda, tripple dips on reducing unhappiness, and usually the build time is like 1.5 - 2 building, so considering how few there are pure happiness buildings, it is worth it.
Also, very important is to manage your friends, you should always, make your likes and dislikes based on long term strategy, whom you conquer first, there is not relation booster like fightning common war, and if you have frined you too much to refuse to help, they are displeased eventually. Securing friends will give you a strable trade partner for luxuries, not to mention, the more wars they have,
the less partners hey have to trade their excess luxuries. Also, you should always buy exces luxuries, as they become available to trade, since if they sell them, hey will not be available for long time.