Strategies are completely different on each level.
On the easy ones, pick a SCI Civ, prefereably Russia, play against all other SCI Civs, research like mad, and gift your (otherwise useless) comrades into the next era each time, pulling the scientific slingshot to get a second tier tech.
On the high levels, you can be sure the AI is helpful for research, or fueling your research with cash; also, you can be sure the AI will research several techs for you (namely Nationalism, Invention).
Propably the hardest time is Regent/Monarch; here, the AI is a bit unpredictable, so you cannot tell what (and if at all) they research. OTOH, they have no substantial cash to offer. So here, it is indeed 'milking' at max; get huge, use lots of Scientists etc.
But, you will launch faster with 25% land on Chieftain as well as Deity than you would with 50% on Regent

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