Should LaTeX be used for all expressions, including simple ones?

Not necessarily. If the numbers, symbols, and letters needed for an equation are available on a standard keyboard and can be entered in a clear manner, then LaTeX is not required. Instead, LaTeX can be reserved for complex formulas and equations, including fractions.

A previous version of the Pressbooks User Guide made this suggestion:

If LaTeX is a bit much for you or you’d rather look at other options, you can always use special characters, symbols, and the sub/superscript buttons on the toolbar to compose your equations.

If you are fluent in HTML+CSS, you can also look at constructing and formatting your formulas that way.