For using a NIF you do not want to use a language that has a GC, because it will conflict with the Erlang BEAM GC unless you are very very very careful about where data is ‘owned’ and that you prevent the GC from running on any of the BEAM threads, but as long as you can verify all that then it’s fine.
On a crystal side I messed with it for a couple small tools, doesn’t seem bad but not really my style. For something C’ish like that I’d go more for Nim or so (though for most things I’d go Rust nowadays anyway). Plus when I last tried it, it still had a GIL, so no parallelization.