I use the package mathrsfs to get a separate \mathscr. While I like the way it looks, it does not come with a bold version. More specifically, the usual \mathcal is compatible with \boldsymbol and \boldmath (see this question), whereas \mathscr is not:
\documentclass{article}
\usepackage{mathtools}
\usepackage{mathrsfs}
\begin{document}
\[
\mathcal{S}, \mathscr{S}.
\]
\[
\boldsymbol{\mathcal{S}}, \boldsymbol{\mathscr{S}}.
\]
\end{document}
Is there an alternative package that provides similar looking letters to mathrsfs, but which does come with a bold version?

mathalphapackage. Its documentation contains some lists of calligraphic fonts including some which come with a bold variant. – campa Apr 21 '21 at 12:23