①の文章:
"Everyone should not take things so seriously"=「皆は物事をそれほど真剣に考えなくても良い」
②の文章:
"Everyone should not think of it so seriously"=「皆はそれに関してそれほど真剣(重く)考えなくていい)
"everyone could think of like casual better"という言い方は語順を少し変えた方が、文法的に良いと思います→ "It would be better if everyone could think (about things) more casually"