I was very surprised to see wiktionaries phonetic transcription and sound sample of "Buch", apparently it is not throaty everywhere.

It's a /k/ in all "-achs", e.g. "Dachs", "wachsen", "Flachs", exactly as in "bochs" the emulator (compare "boxen"), except across boundaries, e.g. "wachsam, wach-sam" (wakeful, at guard), or contractions, e.g. "[Meister seines].Faches/Fachs".