Setting tax rate

I'm pretty sure you can only set tax rates globally, meaning you can't do it for specific planets.

I think the developers felt it would be too much micromanagement to manage taxes on a planet by planet basis, especially on larger maps.

Skunkybro is correct on both counts.

What can help is going into the list of planets from the civilization tab and sorting them by happiness.

Adjust your taxes for the desired effect and watch the happiness rates change on all the planets- For planets that are too low, add extra entertainment buildings there if possible.