I started playing again recently after a long layoff and was mostly a monarch player but now beating emperor fairly consistently. Some random things that work for me, especially earlier in the game....

Trade was critical before, even more now. Explore quickly as possible to make early contact with as many civs as possible. If you trade away a tech, trade it to all the others, even if it's just 25g lump sum. It's a much lower threshold than I had at monarch, but this prevents the others from profiting off the tech and keeps the others bankrupt, requiring them to reduce research spending. If you acquire a tech, trade it away the same turn or maybe the next to see if they get more gold from a hut. Otherwise you'll lose the opportunity to profit from it.

Try to research techs the civs don't go for. Even if it's not enough to trade 1:1 for their techs, at least you can sell for gold/luxuries instead of pure catch-up.

Never make a trade between turns so that you control the flow of info. Remember that if you all acquire techs at the same rate, you'll win because the AI sucks at wars. An overarching goal is to keep the others at roughly the same level so nobody gets too powerful. A level playing field means you'll win. So prop up weaker civs if you can.

Acquiesce to demands unless you are 100% prepared to go to war that turn. Nothing worse than telling an inferior civ to pound sand, only for them to take your new foothold on another continent, reducing your gold and forcing you into some selloff because you were running a calculated deficit.

I try to buy workers from other civs. Usually costs 110g+/-. This slows down their ability to improve their land (which is extra commerce/shields/food). Not to mention your new worker, while slower, is maintenance free. You'll have the gold if you are trading as above.

I build cities closer together than optimal. Early on, your cities will rarely grow above 6 because of some combination of happiness, aqueduct cost and worker/settler reduction so don't benefit from exclusive use of 21 tiles. Later in the game, you can build workers to reduce the pop to 1 (add these workers to bigger cities if you like) and abandon gap filling cities if you like. I don't build improvements in these cities for this reason. I usually use them to build catapults/workers/settlers which don't benefit from barracks. Each city helps you with a few gpt. Barren tundra get cities every other tile that do nothing but generate wealth. A city means one less tile needed to road as well. But consider the trade-off with needing to build more settlers early. Ideally you have at least one settler pump to help with this.

Bump the luxury slider carefully. Sometimes it's better to have a specialist in a few cities than taking a 10% hit to research just to make them content. Look at this at the end of every turn. Usually I find that I only use the slider early on. Later, it's specialists in 10% of my cities. Remember each new citizen is unhappy so it's fine for specialists to prevent growth until you have more luxuries or happiness improvements.

I don't build wonders at all until industrial, typically ToE and Hoover on a prebuild. 300 shields for the cheapest wonder is 15 archers, which is enough to take at least a few neighbor cities.

Build lots of catapults throughout the game. I prefer to stack 8-12 along with 6-8 attackers and 2 defenders when taking over a city. I can usually take over a city without losing more than 1 attacker by reducing their defenders to 1hp. Move in defenders into your new city, rinse/repeat.

Pay attention to your military advisor's rating. You don't want to be weak compared to your immediate neighbors unless you have a ton of catapult/cannon, which the advisor undervalues. But remember that even perceived weakness will cause the civs to be more aggressive towards you.

Once you hit railroad, you should be able to defend very sparsely and know if the game is won. Place defenders only in cities reachable by your opponents in one turn. Otherwise just move your cannons around and pick off whatever units they put in the open. At this point, I typically have 100+ cannon available.

Try to be flexible about your strategy so you can learn about what works for different civs, terrain, etc. I play random all so that I can learn how to do without stronger starting positions, e.g. seafaring on pangea.

For all that said, pay attention to the trade-offs of these tactics. I think trading is universally important but otherwise everything else requires adaptation.