Columbus's arrival in the Caribbean, by sheer luck, hastened the conquest of Mexico significantly. Without his stupid ideas about the circumference of the Earth, it's quite likely that European colonization in the early sixteenth century would have happened only in Brazil and the northeast.
I commend you for asking the question and adding a poll as I think I have done in the past. I commend you even more for getting a spirited response from folks!
I disagree : Portguese explorers will map the coastline of the Americas in a few years (because why not) and even if they try to keep a secret, it will get out and by probably 1510, Europeans will know how huge this land mass is.
Does it really have to leak *that fast*?
---responding to the poll, I voted for a late one, 1661. Mainly I was just trying to be provocative. But let's say the discoveries of Newfoundland and Brazil are delayed beyond OTL. Not too hard. Cabot was probably only funded by Henry VII because Columbus showed the way. Fishermen's tales can take much longer to penetrate into royal minds and get people thinking of state-sponsored voyages in that direction.
Brazil could be delayed. In OTL the Portuguese accidentally bumped into Brazil on their second voyage to India. Now I don't know how many trading fleets the French sent to Africa every year, but the bump into Brazil could be easily butterflied away. Now say there was one Portuguese trade fleet per year every year after the Vasco Da Gama landfall in India in 1497. Now with that volume of fleets, it becomes *very* likely that at least one sites Brazil and comes back to tell the tale by 1507 (by the tenth fleet). It becomes *almost certain* that this will happen before 1517 (and 20 years of fleets rounding the Cape).
But as the OP pointed out, Newfoundland and Brazil do not scream out "El Dorado of gold", follow up exploration will be slower and less expansive, and meanwhile the Indian Ocean and East Asian coasts will be getting better and better mapped and European countries will be competing there and driving each other to greater efforts (Spain coming in as the first challenger, followed up by others eventually).
The big driver of further exploration will be sugar cultivation and the profits it makes. The Portuguese will start sugar plantations on a small scale from whenever they first get to Brazil. They will scale up a lot in a generation. Here's how wiki said it developed in the years after Brazil's discovery (1500) -
"The Portuguese took sugar to Brazil. By 1540, there were 800 cane sugar mills in Santa Catarina Island and there were another 2,000 on the north coast of Brazil, Demarara, and Surinam."
"The approximately 3,000 small sugar mills that were built before 1550 in the New World created an unprecedented demand for cast iron gears, levers, axles and other implements. Specialist trades in mold-making and iron casting developed in Europe due to the expansion of sugar production. Sugar mill construction sparked development of the technological skills needed for a nascent industrial revolution in the early 17th century.[29]"
So, by that point people are going to start heavily exploring the American coasts in the tropical areas simply to scout out possible sites for sugar cultivation, which will lead them quickly to the Caribbean.
A parallel driver of exploration, and settlement in particular, will be religious conflict and persecution. That is where the Reformation and Wars of Religion come in. Once Newfoundland is mapped, by the time you have religious wars occurring with frequency and at scale (by 1550 at latest), you will have people willing to explore and settle in northeastern North America. If Brazil or the Caribbean sugar boom attracts multiple colonizers to Brazil (likely) and the Caribbean (definitely), religious dissenters will seek to live in territories there, where the political situation permits (in Huguenot, English or Dutch colonies). Once the Caribbean is explored, the discovery of Mexico is going to come within 15 years tops, and conquests on some scale will begin shortly after.