Unless you are very careful the endgame becomes an enforced "rush" of the remaining provinces.
Expansion eventually all but eliminates trade income; you do not trade with yourself or enemies. Farm income from your empire will partially offset this if you have upgraded farms as you expand. You can trade with Rebel ports if you are not at war with the Rebel faction.
As income declines you need to be more efficient in allocating forces. You might want to disband anything not needed. If your treasury is really hurting it would be worthwhile to simply examine the map and devise a plan for the quickest wind-up of the game...invade/sack/stabilize/next.
Good Luck! :)
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