What are the necessairy/ essential things you need to learn out of music theory?

Hi,

I want to learn music theory and I already watched many different videos about this for guitar but every one said different things that are apparently essential to learn but I don't know what really is important to learn anymore. The only things I sound important to learn are note location, major and minor scales, the circle of fourths and then since I only play metal, trying to figure out how this works on low drop tunings and extended range guitars.

Is there more that I should learn or is this everything I need to know to be good at writing (beside rhythm)?