Updated version: code now works, added some whitespace to make some things more obvious.

mwe-thesis.tex

\documentclass{minimal}
\usepackage{filecontents}
\begin{filecontents}{thesis.tex}
\documentclass{book}
\input{preamble}
\begin{document}

    \chapter{Lit Review}
    \input{litreview}%

\end{document}
\end{filecontents}
\begin{filecontents}{preamble.tex}
\usepackage[english]{babel} % just for the MWE
\usepackage{blindtext} % just for the MWE
\usepackage{standalone}
\usepackage{tikz}
\usetikzlibrary{mindmap}
\end{filecontents}
\begin{filecontents}{litreview.tex}
\documentclass[preview]{standalone}
\input{preamble}
\begin{document}

    \blindtext

    \begin{figure}
        \input{varieties}%
        \caption{Caption}
    \end{figure}

    \blindtext

\end{document}
\end{filecontents}
\begin{filecontents}{varieties.tex}
\documentclass[tikz]{standalone}
\input{preamble}
\begin{document}

    \tikz[mindmap,concept color=red,text=black,grow cyclic]
        \node[concept] {Traditional}; % [clockwise from=0]

\end{document}
\end{filecontents}
\begin{document}
Placeholder content.
\end{document}