Absolutely right. Nobody really wanted to sponsor Columbus because they already knew how big the globe was, and that Columbus' proposed expedition would run out of supplies before he got to east Asia by going west - because they were unaware of the Americas existing.

The earliest globe that still exists was started in 1490, and finished in 1492, the very year that Columbus set sail. Columbus didn't return to Spain until March 15, 1493.

as far back as the 6th century

At least 800 years before that, maybe more. Eratosthenes calculated Earth's circumference around 240 BC. I would assume you have to believe the Earth is round is if you're measuring a circumference.

You're right, many (most?) ancient civilizations knew the earth is round.

It's not a big mental leap. Things disappear over the horizon. The sun and moon are round. Maybe the earth is round too 🤔

I always wondered where the crazy idea that people once believed that the Earth was flat came from.

So, looks like it comes from a single prolific academic that mixed fiction and history, describing people just about a century before him, and that everybody took for gospel. A famous book said "hey, everybody on your grandpa generation believed this", and it became common knowledge!

I'm not sure I'm even surprised by something like this.

Yes, Eratosthenes estimated the circumference of the Earth "with an error on the real value between −2.4% and +0.8%", probably sometime between 250-200 BC (his adult life - I don't think we know the exact date when he published his work).