How to conditionally include sections/formats from command line?

The package multiaudience (https://ctan.org/pkg/multiaudience) may be exactly what you are after. It uses environments (or, at least they look like environments) to determine which parts are conditional.

From the multiaudience document (describing the different audiences of 'developers' and 'executives'):

You may set this parameter outside the document itself like this:

pdflatex "\def\CurrentAudience{execs}\input{file}" This trick allows one to generate all versions of output from the command line:

pdflatex -jobname file-execs "\def\CurrentAudience{execs}\input{file}"

pdflatex -jobname file-devs "\def\CurrentAudience{devs}\input{file}"

Which means that your makefile will need more targets, but I assume you have that in hand.

For your use-case, you would need to define the audiences, and then use something like this in the document:

\begin{shownto}{onepage,multipage}
  \input{resumesection_education.tex}
  \input{resumesection_experience.tex}
  \begin{shownto}{multipage}
    \input{resumesection_projects.tex}
    \input{resumesection_extracurriculars.tex}
  \end{shownto}
\end{showto}

Anything outside of the showto command or environment should be unconditionally included, so it may also be possible to be closer to your example:

\input{resumesection_education.tex}
\input{resumesection_experience.tex}
\showto{multipage}{
    \input{resumesection_projects.tex}
    \input{resumesection_extracurriculars.tex}
}