An independent California and Cascadia wouldnt be any less settler states than the US is. The issue with the US is that the actual native peoples have been so thoroughly eradicated that to remove the settlers entirely would depopulate the nation.
What i would do personally is once the US govt thats in place now is overthrown get all the tribal leaders and representatives of other native communities together and work with them to find a good solution.
I personally would think a good system could be one where each native tribe has a garunteed seat in any governing body and has a heavily weighted representation. Like how some states now have more say per person in the senate.
You could pair that with higher autonomy for tribal lands and giving them more land, access to seaports, things like that. And on the individual level offering support to native peoples. Especially ones who want to have kids because there are literally less native americans today then there were before colonization. Like a lot less.
Another big thing is just listening to native peoples about land usage. California wouldnt have the wildfires it has now for instance if it had listened to native peoples advice on how to manage the land.
As for balkanization itll probably be inevitable but it will make doing these things harder. I imagine the US will have a similar path as China. Where it fractures then communists gain a foothold somewhere and slowly reunite the country.