After Alexander the Great’s death the Late Period was definitely over, and Egypt was ruled over by his general Ptolemy and his successor in what we now call the Ptolemaic Period.
This period lasted from the accession of Ptolemy I in 323 BCE through until the death of Cleopatra VII in 30 BCE. It can be confusing to read about this era – all the male rulers were called Ptolemy, the women chose between Cleopatra, Asinoë and Berenice. And in general they all married each other, often co-ruled with siblings or parents and fairly frequently murdered each other.
Egyptian culture survived during this time – the Ptolemies ruled as Pharaohs and the ancient religion continued. Many of the temples that you visit on a typical Nile cruise are from the Ptolemaic Period, in large part because these were the last to be built and no-one came along later to refurbish them to suit new fashions. But the Ptolemies were still definitely foreign rulers – Greek citizens were a privileged elite, Greek became the language of the upper classes and the administration, and Egyptians were second class citizens.
I’ve chosen Cleopatra VII’s death as my end point for what “counts” as Ancient Egyptian history. You can make a case for Alexander the Great’s conquest as the last moment, but I feel the Ptolemies had some desire to remain continuous with the idea of Pharaonic Egypt. You can make a case that Roman Egypt still has cultural continuity, but I don’t think the Emperors of Rome were really interested in being Pharaoh. And from a narrative perspective it’s a satisfying ending – this great civilisation ends with a bang, not a whimper.