\documentclass{scrbook}
  %\usepackage{luatexja}
  \usepackage{fontspec}
  \usepackage{luatexja-fontspec}
  \usepackage[
    mode=text,
    detect-all,
]{siunitx}
  %\usepackage{luatexja-fontspec}
\makeatletter
\typeout{\meaning\@begindocumenthook}
\begin{document}
  \ifdefined\gtfamily\gtfamily\fi Ω
  {\ExplSyntaxOn\makeatletter
   \ttfamily\meaning\@begindocumenthook}
  
  \SI{-2.5e-10}{mol.L^{-1}}\SI{1}{\micro\metre\angstrom\arcminute\arcsecond\ohm}
  \vrule\SI{}{\ohm}\vrule
  \end{document}