If you're not making heavy use of corporations, you could switch to state property. Eliminates distance to palace and colonial upkeep costs completely, as well as boosting workshops and watermills - in itself good for a domination victory, since more hammers means more units. Otherwise, you can also spin off all cities you own on a particular landmass as a colony, which becomes your vassal, or make your enemies capitulate to you and then gift them back the cities you conquered. Either way, this means you get half the population/land value of these lands to count towards the domination criteria, without worrying about maintenance costs.