Classically that’s true, but if we want consistency with quantum mechanics, the only starting point we know is to quantize the force carrying fields (gravitons,

Classically that’s true, but if we want consistency with quantum mechanics, the only starting point we know is to quantize the force carrying fields (gravitons, just like photos and gluons), but then it turns out we can’t get far from the starting point. Why do we want to quantize gravity? Imagine you are doing a scattering experiment. To predict the full results consistently (in principle), you would need to include the gravitational forces between the scattering particles. This would be particularly important in early universe cosmology where you might have lots of heavy stuff zipping around at high speed! The classical GR picture is not too helpful in these situations.