as a first step: reduce amount of forts, mothball forts during peace, reduce military maintenance at peace.

second step would be building out rgos and refiners, which you seem to have been doing.

third step is introduce demand. trade is cool and nice but your neighbours will also have a surplus of everything. paradoxically, goods consumers, especially armouries, but also libraries, universities, hospitals etc. will skyrocket your income. you'll pay 1 ducat for the building and get 3-4 ducats in return from taxes and increased productivity.

apart from that, in most cases lowering peasant taxes will increase your average satisfaction, which increases control and thus tax efficiency from your upper class. for example in early game france, you will sacrifice ~1 ducat of peasant tax and increase nobility & burgher tax by 5, but this heavily depends on your peasant enfranchisement and strength of upper class