There is no need to load bm with stix; the fonts come with bold letters that works with the standard \mathversion{bold} and \boldsymbol from amsmath.
With PDFLaTeX, you can load a bold sans-serif math alphabet with isomath. There are several options, including Computer Modern Bold and Arev.
\documentclass{article} \usepackage{iftex} \ifTUTeX \usepackage{unicode-math} \setmainfont{XITS}[Scale=1.0] \setmathfont{XITS Math}[Scale=MatchLowercase] % Also loads STIX Math Bold. \else \usepackage{amsmath} \usepackage{stix} % sfdefault=zavm is Arev. sfdefault=cmbr is Computer Modern Bright. \usepackage[sfdefault=cmbr]{isomath} % For \mathsfbfit \newcommand{\mbfitsansalpha}{\mathsfbfit{\alpha}} \newcommand{\mbfitsanslambda}{\mathsfbfit{\lambda}} \fi \pagestyle{empty} % For convenient cropping of the MWE. \begin{document} \begin{align*} \alpha &= \lambda \\ \boldsymbol{\alpha} &= \boldsymbol{\lambda} \\ \mbfitsansalpha &= \mbfitsanslambda \end{align*} \end{document}
Compiled with PDFLaTeX, this gives:
With unicode-math, this is even simpler: the package supports both \symbfit and \symbfsfit alphabets out of the box. The XITS Math font by Khaled Hosny additionally comes in a bold version.
The same MWE compiled with LuaLaTeX or XeLaTeX produces:
The range= option of unicode-math allows you to substitute the sans-serif math alphabet from any other math or Greek font. Here is an example of how to substitute the Greek sans-serif italic alphabet from STIX Two Math.
Unfortunately, as of 2020, range= is not compatible with math versions, so you would have problems with \boldsymbol. I therefore switch to \symbfit.
\documentclass{article} \usepackage{unicode-math} \defaultfontfeatures{Scale=MatchLowercase} \setmainfont{XITS}[Scale=1.0] \setmathfont{XITS Math} \setmathfont{STIX Two Math}[range=bfsfit/{Greek,greek}] \pagestyle{empty} % For convenient cropping of the MWE. \begin{document} \begin{align*} \alpha &= \lambda \\ \symbfit{\alpha} &= \symbfit{\lambda} \\ \symbfsfit{\alpha} &= \symbfsfit{\lambda} \end{align*} \end{document}
You're more likely to want to substitute a different bold italic lambda (𝝀) symbol here, though, with range=\mbfitlambda.
If you genuinely care about portability between isomath and unicode-math, \mathbfit works in both packages if you give unicode-math the option bfit=sym, but the equivalent of \symbfsfit in unicode-math is \mathsfbfit in isomath.
