Commerce is very powerful and you can earn more money from commerce than taxes.
To obtain the best return for commerce, the player has to maximize exports as they have a x1 of trade value. Imports or national trade are not that important (0.35 and 0,20 respectively) for income.
To ensure that you have enough goods to export, the player has to invest in mines and slaves estates in settlements and forges and mills in cities with the best trade goods.
To guarantee that you have enough buyers for your goods, the best thing is to be centered in the map to have enough trade partners. If this is not possible or you do not have enough of them, vassals and league cities are your friends.