Tips to Deal with Low Cohesion
Usually, the main contributor to low cohesion is inequality, so welfare is likely your best bet in the long term. But it can take a long time. Many nations need a few decades.
If your nation is high in unrest, too, I recommend doing stabilize first, until it’s below 2.0. Higher unrest drastically reduces montly investment points, and thereby your investment programs will be hampered.
A good short term solution might be declaring rivalries with near-peer nations (i.e. same size or larger). You get +.5 cohesion per near-peer rivalry, as long as your nation and/or the rival isn’t a democracy.
Dealing with cohesion in mega-nations eventually will just mean using enough Unity to keep it where you want it. The Cohesion resting point is calculated, in part, based on how big the country is, so a sufficiently-large nation won’t be able to raise it above 0 even at minimum Inequality. But, a nation of such size can afford to use enough Unity to counteract the monthly cohesion loss.