You say that a worldwide flood is impossible, but I say that if the land mass of the Earth were perfectly spherical,
This is based on purely speculation (yours) and false information that the Earth is a perfect sphere. It has never been a perfect sphere. It is "what-if" that never was, and never happen.
the water would cover the Earth and be 2.5 km deep everywhere (excluding calculations for tides).
Again, more "what-if" scenario that didn't happen.
For there to be a global flood, like the way Genesis say, there need to be both geological evidences, archaeological evidences and even biological evidences (such as mass extinction of those animals that didn't make it, to board the Ark, hence all death by drowning), EVERYWHERE, AT EXACTLY THE SAME TIME.
We have many evidences of ancient floods, because they occurred annually in many places. Archaeologists for instance, can tell which city suffer from such as fire, earthquakes, or floods.
No where in human history are there any evidence for a global flood, that lasted a whole year (so the Genesis claim), not geologically, not archaeologically and not biologically.
No where in geological history does it provide a single evidence of global flood during the Bronze Age or the Neolithic period.
And if there was such a global flood, it should have caused a break in civilisation or culture. Logically (hypothetically), the civilisations and cultures AFTER the flood (after 2340 BCE) should be very different from the civilisations and cultures BEFORE the flood (before 2340 BCE).
Meaning, the styles in arts (paintings, sculptures, pottery, architecture, etc) would be different from the one before the global flood. Writing systems before the flood would be lost, and the ones that come afterwards should be different. The customs before and after the flood should be different.
If the Flood had happened, hypothetically, it would have taken place between 2400 and 2100 BCE (early Bronze Age in Egypt and the Near East), by using the calculations of all the years, generations and reigns mentioned in the bible, to a specific point of actual and verifiable history the fall of Jerusalem in 587 or 586 BCE. Counting backward all those years, that the Flood would have happened in 2340 BCE, if the calculations were based on the (Hebrew) Masoretic Text (compared to the Greek Septuagint manuscripts).
And if everyone but Noah's family had died, then it would have taken time, at least 20 years, for the children of Shem, Japheth and Ham to reach maturity. But looking at Shem's genealogy, Shem's successor, Arphaxad (supposedly the middle child of Shem's 5 sons), was born 2 years after the flood. Unless, Arphaxad was a twin with Asshur, I don't see how could even be possible.
2340 BCE would have hypothetically put the Flood in the reign of Teti I (2345 – 2333 BCE), the 1st king of the 6th dynasty in Egypt. Had there being a flood, he should have been killed, and yet here we have a king, building pyramid at Saqqara, like his predecessors, including Unas, the last king of the 5th dynasty. Teti and his son and successor, Pepin I (who have his own pyramid built for himself), were still using the same writing system as all his other predecessors (Egyptian hieroglyphs and hieratic). There were also no change in styles of arts, pottery and architecture.
Culturally, Egypt have been around during the Neolithic period. This can be seen in old artefacts that depicted gods and goddesses (eg Horus, Seth, Hathor, Neith, etc) dating before the 1st dynasty, known as the Predynastic period (, when Egypt was actually two kingdoms.
But in Genesis 10, Egypt didn't exist, until after Noah family began multiplying. Egypt is supposedly the son of Ham. That would mean for at least 20 years, there were no Egypt until Ham's son came to age (adult). 20 years may not seem a long time, but it would be noticeable for the size of the kingdom.
And Egypt isn't the only kingdom or civilisation showing that Noah's deluge didn't happen.
2340 BCE would hypothetically put the flood during very late Early Dynastic period (c 2900 - c 2350 BCE), just before the founding of the Akkadian empire (c 2350 - 2193 BCE).
Sargon the Great (reign 2334 - 2279 BCE) was the founder of Akkadian dynasty in 2334 BCE, and founder of the city Akkad or Agade.
Akkad is the same city that supposedly built by Nimrod in Genesis 10:10, the son of Cush and grandson of Ham. Nimrod supposedly also found Babylon and Uruk.
Nimrod is a historical figure, Nimrod isn't.
Another reason for rejecting anything and everything in Genesis, is that of the city of Uruk. Genesis 10:10 claimed that Nimrod had Uruk built (called Erech in the KJV translation). But historically and archaeologically, Uruk is much older than 2340 BCE. During 2900 to 2600 BCE, Uruk was a thriving and important Sumerian city, in which Gilgamesh was supposed to be a king of Uruk in the 27th century BCE.
But Uruk is even older that. Around 5000 BCE, the first Neolithic settlement is found in this site. Uruk kept growing in size and in importance during the throughout the 5th millennium BCE, known as the Ubaid period (6800 - 3800 BCE), Uruk was a modest size town.
By the 4th millennium BCE, became a full size city, the largest in the world at that time, and all before the Sumerian civilisation. It reached its height during 3500 - 3200 BCE, where they have several temples, especially to the sky god An (Anu in Akkadian and Babylonian) and to the sky goddess Inanna (Ishtar in Akkadian-Babylonian). And even earlier cuneiforms writing exist in Uruk before the Sumerian cuneiforms, known as Proto-Sumerian cuneiforms.
This show that the author of Genesis didn't the history of pre-Sumerian and Sumerian Uruk, just as it doesn't know much about Egypt.
Believing in the bible, particularly with the Genesis Noah and the Flood are taken on as blind faith. And taking the bible as historical records is not only pointless, it is unreliable source of information, no better than relying on the epic of Gilgamesh or Hesiod's Theogony.