Kodi does have an option to use its own fmt, if that ends up being necessary. The latest git definitely has support for fmt 9.1.0, so it may be an issue of needing to wait for a release that supports it (or apply the relevant patch, as the commit that updates it changes one line).
It seems like a big thing would be updating the relevant software, as most of them seem to have been adapted in newer updates if they don't just work.
@Phantop: You could find a single patch to fix the build for Kodi with fmt 9.1.0? Asking because I got into a bit of a patch mess yesterday when I took a look at it 😆
FWIW I'm all for a simple patch. They're at beta 1 for their next release now, which could still mean two months (or more) until the final release, if you go by past schedules.