So what’s the best subject to learn to write and think

English, with minors in logic and and rhetoric, perhaps? I took maths and physics, and the problem with science courses used to be that you were taught to write in a very stodgy way using the third person and passive voice, but they may have improved since 40 years ago.

The best way to learn to write is to write. You could practise by writing detailed answers on Reddit. Make them mini-essays, and limit yourself to so many words.

The Pyramid Principle, by Barbara Minto, will teach you how to structure your ideas and prose, although it is hard going at first, and it is focused on technical and business writing. It won't teach you how to write novels.