Find the 5 most valuable trade provinces left. Then pick them off using naval attacks. Invest in internal infrastructure for wealth and happiness as at some point all trade will stop.

You are almost at the point where everyone will start fighting you. So think about taking over a corner of the map and minimising your frontage. Remember that rebellions are going to be your biggest risk. So do your best to make the people happy and have counter rebellion armies spaced out through your larger and larger empire.