What is the ruling on one who commits an invalidator of Islam: does he return to Islam by repentance alone, or must he perform ghusl and utter the Two Testimonies? And do his previous good deeds become void? And is delaying prayer beyond its prescribed time considered disbelief? And is it permissible for one who has delayed a prayer to pray the subsequent prayer without repentance?

It seems the questioner is suffering from whispers (waswasa) related to takfir (declaring someone an apostate), and they should ignore these thoughts. Takfir is not a light matter, and a Muslim is not judged to have exited Islam unless all conditions are met and all impediments are removed. If a Muslim apostatizes, they must pronounce the Shahadatayn to re-enter Islam, and then perform ghusl (ritual bath), though there is a difference of opinion regarding its obligation. The preponderant opinion is that apostasy does not nullify previous good deeds unless the apostate dies upon their apostasy, even though the reward of deeds is nullified by apostasy. This applies to every apostate, such as one who abandons prayer while denying its obligation. As for one who abandons prayer out of negligence, some scholars consider them a fasiq (sinner) (the view of the majority), while others consider them a kafir (disbeliever). Deliberately abandoning prayer is more severe than some major sins, and the questioner must repent and make up for the prayers they missed.