Be careful not to conflate special (SR) and general relativity (GR). SR has nothing to say about gravity, and, as you point out, has already been successfully m

Be careful not to conflate special (SR) and general relativity (GR). SR has nothing to say about gravity, and, as you point out, has already been successfully merged with Quantum mechanics to form relativistic quantum field theory (Dirac's early versions of which led to the proposal of antimatter). It is GR that describes "what gravity does", and which is difficult to enmesh with quantum mechanics.