Then the change of field strength is explained. The field strength can be expressed as a negative gradient of gravity potential (gravity potential U=-GM/r). Considering that the whole field distribution is spherically symmetric, the gravitational field strength e =-du/dr. Because the gravitational potential inside the spherical shell is constant and the outside is hyperbolic, the point on the spherical shell is non-conductive, and the gravitational field strength at this point is not defined mathematically and does not exist physically. Just like your calculation "gravity is either 0 or GMm/R*2", there is no error, the field strength at this point does not exist, is the change of gravity continuous?
From a real point of view, any point in space should have a gravitational value (regardless of some corrections and special circumstances under the condition of general relativity), so similarly, the spherical shell model is also a mathematical abstraction, that is, there is no object with zero thickness and mass in the world, as long as there is a thickness field strength, it is continuous.