“I believe the idea that Columbus thought there was a westward route to India is an urban legend.”
It was his stated purpose when seeking funding for the trip.
Also, to enhance his discovery as fulfilling the terms of his financing, he named the local he found Indios (Indians).
He might not have been beyond bending the facts a bit. Because of differences in how star observations correlated with the ship clock, men already had reason to believe a round world larger than what we know as the Old World.
I could be wrong, but I believe that Columbus severely underestimated the size of the earth. I think that fact is why the experts of the time rejected his idea—he thought that Asia was 3-4000 miles west of Europe. They knew, or at least had a good idea of the true size of the earth. There is no way he could have sailed the 12,000 miles to China.
No living European of his time had seen a Chinese/Indian. He knew from Marco Polo’s book that Chinese had straight black hair and very little body hair. They spoke a language that no one in Europe understood and all kinds of exotic flora and fauna. When he landed in the Caribbean he found everything he expected to find—except the great civilizations.