What you are looking for is the Born-Oppenheimer Born-Oppenheimer approximation, which is exactly what is done when solving for the electron part and the nucleus part of the Schrödinger equation separately. You can read more about assumptions and the approximations in the wikipedia atricle, but the core is that the nuclei are far heavier than the electrons and therefore move more slowly

What you are looking for is the Born-Oppenheimer Born-Oppenheimer approximation, which is exactly what is done when solving for the electronic and nuclear parts of the Schrödinger equation separately. You can read more about assumptions and the approximations in the Wikipedia article, but the core is that the nuclei are far heavier than the electrons and therefore move more slowly.