Are internal thoughts that involve cursing religion considered disbelief (kufr), and do they require uttering the Shahadah and performing ghusl to re-enter Islam, or are they from the whispers of Satan?

Cursing the religion is apostasy that necessitates repentance. One must guard their tongue from falling into it. There is disagreement regarding the necessity of ghusl (ritual bathing) in this case. As for what crosses the mind but is not uttered, there is no accountability for it.