Do We Already Have a Grand Unified Theory?

If we do, it's a non-mainstream one; yes, that usually matters, in terms of speed of acceptance. Burkhard Heim and a few disciplies claim that he resolved the inconsistencies between relativity and quantum mechanics. The math is beyond my poor power to evaluate it but doing a "social meta-analysis" of Heim's behavior, he certainly doesn't fit the standard physics crank profile as developed by Pascal Boyer, first and foremost because Heim developed a new calculus for his theory. (Most cranks can't get past algebra). John Reed abandoned his exegesis of Heim's work after locating some obscure defintions that Reed claims show Heim's work to be circular. Dröscher and Hauser appear to still be developing Heim's ideas; start here for constants and notation.