The corrections generated by the finite size of the nucleus are not only from the possibility of having electrons inside it, e.g., the isotope shift. Pyarelal and Bhatnagar proposed a model of the hydrogen atom with an impenetrable nucleus of finite size and showed that the reduction in the volume available for movement of the electron produces an increase in the energy of the electronic states.
More information can be found on:
Aquino, N., Rojas, R. A., & Montgomery, H. E. (2018). The energy correction due to a finite size nucleus of the hydrogen atom confined in a penetrable spherical cavity. Revista mexicana de fĂsica, 64(4), 399-406.