The best way to reduce the FontAwesome fonts or files is to change the template
font_awesome_setup, so you only request the required font and not all fonts. Your addon is nice, but if you reduce the included fonts there are always issues with third party addons and/or themes.
Look for the template
font_awesome_setup..
[CODE title="Original template"]<xf:set var="$faVersion">5.12.1</xf:set>
<xf:if is="fa_weight() == 'l'">
<link rel="preload" href="{{ base_url('styles/fonts/fa/fa-light-300.woff2?_v=' . $faVersion) }}" as="font" type="font/woff2" crossorigin="anonymous" />
<xf:elseif is="fa_weight() == 'r'" />
<link rel="preload" href="{{ base_url('styles/fonts/fa/fa-regular-400.woff2?_v=' . $faVersion) }}" as="font" type="font/woff2" crossorigin="anonymous" />
<xf:elseif is="fa_weight() == 's'" />
<link rel="preload" href="{{ base_url('styles/fonts/fa/fa-solid-900.woff2?_v=' . $faVersion) }}" as="font" type="font/woff2" crossorigin="anonymous" />
</xf:if>
<xf:if is="fa_weight() != 's'">
<link rel="preload" href="{{ base_url('styles/fonts/fa/fa-solid-900.woff2?_v=' . $faVersion) }}" as="font" type="font/woff2" crossorigin="anonymous" />
</xf:if>
<link rel="preload" href="{{ base_url('styles/fonts/fa/fa-brands-400.woff2?_v=' . $faVersion) }}" as="font" type="font/woff2" crossorigin="anonymous" />[/CODE]
and change it to..
[CODE title="Changed template"]<xf:set var="$faVersion">5.12.1</xf:set>
<xf:if is="fa_weight() == 'l'">
<link rel="preload" href="{{ base_url('styles/fonts/fa/fa-light-300.woff2?_v=' . $faVersion) }}" as="font" type="font/woff2" crossorigin="anonymous" />
<xf:elseif is="fa_weight() == 'r'" />
<link rel="preload" href="{{ base_url('styles/fonts/fa/fa-regular-400.woff2?_v=' . $faVersion) }}" as="font" type="font/woff2" crossorigin="anonymous" />
<xf:elseif is="fa_weight() == 's'" />
<link rel="preload" href="{{ base_url('styles/fonts/fa/fa-solid-900.woff2?_v=' . $faVersion) }}" as="font" type="font/woff2" crossorigin="anonymous" />
<xf:else />
<link rel="preload" href="{{ base_url('styles/fonts/fa/fa-solid-900.woff2?_v=' . $faVersion) }}" as="font" type="font/woff2" crossorigin="anonymous" />
</xf:if>
<link rel="preload" href="{{ base_url('styles/fonts/fa/fa-brands-400.woff2?_v=' . $faVersion) }}" as="font" type="font/woff2" crossorigin="anonymous" />[/CODE]
The change is only the if/else request to load just the font you choose in the style properties.