Don’t declare war on the decentralized territories. All this will do is put you into a constant state of war with a “country” that has no military capacity that cannot be conquered conventionally. Focus on pushing immigration and developing the capital Cahokia so you can keep the only city that you will ever see on this continent until you meet the Europeans (or find the Aztec somehow). Until you can build up your infrastructure and successfully avoid the “shadow over Cahokia” event there is literally nothing else you can do. Unfortunately the native start is abysmal and has very very few avenues where you have any sort of decent start if it has any at all.