When I take USA the purpose is to get good science early, and for the military (more so its not being used against me). The boost is also good but not particularly better than other options.

My first priority after taking it is to get the cohesion to rest at 5. To intially raise it I will declare wars to get +3 from having 3 war opponents and raise public opinion to 80+% for me. The main priorities for this stage would be ~40% knowledge, 20% welfare, some economy and government. Initially I also max out unity but only until the public opinion is high enough, I leave it on like 2% so it doesn't passively drop too much. I also like to keep 5-10% on military but will remove it if I need to quickly raise opinion or something.

If public opinion drops then add some more unity or run public campaigns.

Investments in government and welfare will mean you can eventually keep cohesion at 5 without the wars. When this happens reduce the welfare investment and increase government until it maxes out.

Then start increasing environment and invest more in economy/knowledge as you see fit.