[Milhouse]

>I'm not a cmake expert (or even a compilation expert, I mostly just push buttons and hope it works!)
+1
Push button, read logs, try to make sense of cryptic error messages, google it, try to fix, rinse and repeat

Yes, exactly!

>Somehow we need the host build to use the correct $TOOLCHAIN path(s).

Ahh, yes, I stumbled into this long ago.
My solution was to use CMAKE_TOOLCHAIN_FILE for host too, with the right values and pathes of course.
As further improvement to this I dynamically generate CMAKE_TOOLCHAIN_FILE and toolchain.meson hints with a python script, as my build-crap-system is heavily python-based it was trivial to do.

Yes, this seems to be a fundamental issue with the LE "cmake" buildsystem - it's not specific to the flatbuffers package, it's every host package we build with cmake is identifying the "wrong" (ie. host) gcc version rather than the toolchain host-gcc version. It's just that flatbuffers craps out after identifying the wrong gcc whereas other packages build successfully, so I think this needs to be fixed across the board and then flatbuffers should build successfully for LibreELEC. But PR14209 doesn't currently build anyway, so not building flatbuffers is probably the best option for now!

Hopefully one of my colleagues will be able to dig into this - I'll point them at your posts, we already generate the various config scripts but maybe we're missing something vital for host...