Years ago I built a scripting language that transpiled to TCC and then compiled to machine code in memory. It produced human-readable C code so it was very easy to get going: when debugging the compiler I could just look at the generated C code without having to learn any special infrastructure/ecosystem/syntax etc. Plus basically zero-overhead interop with C out of the box => immediate access to a lot of existing libraries (although a few differences in calling conventions between TCC and GCC did bite me once). Another feature I had was "inline C" if you wanted to go low level, it was super trivial to add, too. It was pretty fast, maybe two times slower than GCC, IIRC, but more than enough for a scripting language.

libtcc doesn't give you much control AST wise, you basically just feed it strings. I'm using it for the purpose you mentioned though--scripting language backend--since for my current "scripting-language" project I can emit C89, and it's plenty fast enough for a REPL!

    /* add a file (either a C file, dll, an object, a library or an ld script). Return -1 if error. */
    int tcc_add_file(TCCState *s, const char *filename);

    /* compile a string containing a C source. Return non zero if error. */
    int tcc_compile_string(TCCState *s, const char *buf);