I'm pretty sure there were already in the 19th century people, like Helmholtz, Riemann etc. who saw themselves as Kantians, but nevertheless developed many of t

I'm pretty sure there were already in the 19th century people, like Helmholtz, Riemann etc. who saw themselves as Kantians, but nevertheless developed many of the techniques that later made GR possible (while acknowledging that they challenge Kant's own views) while preserving the core of Kant's theory. See my response above.