Minor adjustment to math font kerning with unicode-math

Question

I would like to use KpMath as my main math font, but also to use the letters and numbers from usual Palatino to match my main text. For this purpose, I replaced the letters and numbers from the font texgyrepagella-math.otf. However, in doing so, the space before the letter f becomes too small:

which should be like

Of course I can fix this in my document by manually add a \, before each letter f. However, it would cause a lot of trouble if one wishes to change the font afterwards. Is it possible to simply modify this spacing?

Below is a MWE.

\documentclass{article}

\usepackage{unicode-math}

\ExplSyntaxOn

\setmathfont { KpMath-Regular.otf }
\setmathfont { texgyrepagella-math.otf }
  [
    range = { it / { Latin, latin }, bfit / { Latin, latin }, up / num, bfup / num }
  ]
\setmathfont { KpMath-Regular.otf } [ range = {} ]

\ExplSyntaxOff

\begin{document}

\( (f) \)

\( f_i f_j \)

\( 123 \)

\end{document}