Resvg seems like the winner here with Firefox's rendering engine being very close too. It's interesting that librsvg is so far behind. I am wondering about the legal perspective of extracting Firefox's SVG engine and using it in FreeType. Or using it in Headless mode as the page you just linked says itself. Also, any performance issues with using Firefox in headless would need to be dealt with. Resvg is looking like a good choice, unless it misses anything essential needed by OpenType SVG fonts, which doesn't seem likely either. But would need to check that to confirm.
Moazin